So... what to do when we want to find a changeset outside of the Doctrine lifecycle? As mentioned in my comment on @Ocramius' post above, perhaps it is possible to create a "readonly" method that doesn't mess with the actual Doctrine persistence but gives the user a view of what has changed.
Here's an example of what I'm thinking of...
/**
 * Try to get an Entity changeSet without changing the UnitOfWork
 *
 * @param EntityManager $em
 * @param $entity
 * @return null|array
 */
public static function diffDoctrineObject(EntityManager $em, $entity) {
    $uow = $em->getUnitOfWork();
    /*****************************************/
    /* Equivalent of $uow->computeChangeSet($this->em->getClassMetadata(get_class($entity)), $entity);
    /*****************************************/
    $class = $em->getClassMetadata(get_class($entity));
    $oid = spl_object_hash($entity);
    $entityChangeSets = array();
    if ($uow->isReadOnly($entity)) {
        return null;
    }
    if ( ! $class->isInheritanceTypeNone()) {
        $class = $em->getClassMetadata(get_class($entity));
    }
    // These parts are not needed for the changeSet?
    // $invoke = $uow->listenersInvoker->getSubscribedSystems($class, Events::preFlush) & ~ListenersInvoker::INVOKE_MANAGER;
    // 
    // if ($invoke !== ListenersInvoker::INVOKE_NONE) {
    //     $uow->listenersInvoker->invoke($class, Events::preFlush, $entity, new PreFlushEventArgs($em), $invoke);
    // }
    $actualData = array();
    foreach ($class->reflFields as $name => $refProp) {
        $value = $refProp->getValue($entity);
        if ($class->isCollectionValuedAssociation($name) && $value !== null) {
            if ($value instanceof PersistentCollection) {
                if ($value->getOwner() === $entity) {
                    continue;
                }
                $value = new ArrayCollection($value->getValues());
            }
            // If $value is not a Collection then use an ArrayCollection.
            if ( ! $value instanceof Collection) {
                $value = new ArrayCollection($value);
            }
            $assoc = $class->associationMappings[$name];
            // Inject PersistentCollection
            $value = new PersistentCollection(
                $em, $em->getClassMetadata($assoc['targetEntity']), $value
            );
            $value->setOwner($entity, $assoc);
            $value->setDirty( ! $value->isEmpty());
            $class->reflFields[$name]->setValue($entity, $value);
            $actualData[$name] = $value;
            continue;
        }
        if (( ! $class->isIdentifier($name) || ! $class->isIdGeneratorIdentity()) && ($name !== $class->versionField)) {
            $actualData[$name] = $value;
        }
    }
    $originalEntityData = $uow->getOriginalEntityData($entity);
    if (empty($originalEntityData)) {
        // Entity is either NEW or MANAGED but not yet fully persisted (only has an id).
        // These result in an INSERT.
        $originalEntityData = $actualData;
        $changeSet = array();
        foreach ($actualData as $propName => $actualValue) {
            if ( ! isset($class->associationMappings[$propName])) {
                $changeSet[$propName] = array(null, $actualValue);
                continue;
            }
            $assoc = $class->associationMappings[$propName];
            if ($assoc['isOwningSide'] && $assoc['type'] & ClassMetadata::TO_ONE) {
                $changeSet[$propName] = array(null, $actualValue);
            }
        }
        $entityChangeSets[$oid] = $changeSet; // @todo - remove this?
    } else {
        // Entity is "fully" MANAGED: it was already fully persisted before
        // and we have a copy of the original data
        $originalData           = $originalEntityData;
        $isChangeTrackingNotify = $class->isChangeTrackingNotify();
        $changeSet              = $isChangeTrackingNotify ? $uow->getEntityChangeSet($entity) : array();
        foreach ($actualData as $propName => $actualValue) {
            // skip field, its a partially omitted one!
            if ( ! (isset($originalData[$propName]) || array_key_exists($propName, $originalData))) {
                continue;
            }
            $orgValue = $originalData[$propName];
            // skip if value haven't changed
            if ($orgValue === $actualValue) {
                continue;
            }
            // if regular field
            if ( ! isset($class->associationMappings[$propName])) {
                if ($isChangeTrackingNotify) {
                    continue;
                }
                $changeSet[$propName] = array($orgValue, $actualValue);
                continue;
            }
            $assoc = $class->associationMappings[$propName];
            // Persistent collection was exchanged with the "originally"
            // created one. This can only mean it was cloned and replaced
            // on another entity.
            if ($actualValue instanceof PersistentCollection) {
                $owner = $actualValue->getOwner();
                if ($owner === null) { // cloned
                    $actualValue->setOwner($entity, $assoc);
                } else if ($owner !== $entity) { // no clone, we have to fix
                    // @todo - what does this do... can it be removed?
                    if (!$actualValue->isInitialized()) {
                        $actualValue->initialize(); // we have to do this otherwise the cols share state
                    }
                    $newValue = clone $actualValue;
                    $newValue->setOwner($entity, $assoc);
                    $class->reflFields[$propName]->setValue($entity, $newValue);
                }
            }
            if ($orgValue instanceof PersistentCollection) {
                // A PersistentCollection was de-referenced, so delete it.
    // These parts are not needed for the changeSet?
    //            $coid = spl_object_hash($orgValue);
    //
    //            if (isset($uow->collectionDeletions[$coid])) {
    //                continue;
    //            }
    //
    //            $uow->collectionDeletions[$coid] = $orgValue;
                $changeSet[$propName] = $orgValue; // Signal changeset, to-many assocs will be ignored.
                continue;
            }
            if ($assoc['type'] & ClassMetadata::TO_ONE) {
                if ($assoc['isOwningSide']) {
                    $changeSet[$propName] = array($orgValue, $actualValue);
                }
    // These parts are not needed for the changeSet?
    //            if ($orgValue !== null && $assoc['orphanRemoval']) {
    //                $uow->scheduleOrphanRemoval($orgValue);
    //            }
            }
        }
        if ($changeSet) {
            $entityChangeSets[$oid]     = $changeSet;
    // These parts are not needed for the changeSet?
    //        $originalEntityData         = $actualData;
    //        $uow->entityUpdates[$oid]   = $entity;
        }
    }
    // These parts are not needed for the changeSet?
    //// Look for changes in associations of the entity
    //foreach ($class->associationMappings as $field => $assoc) {
    //    if (($val = $class->reflFields[$field]->getValue($entity)) !== null) {
    //        $uow->computeAssociationChanges($assoc, $val);
    //        if (!isset($entityChangeSets[$oid]) &&
    //            $assoc['isOwningSide'] &&
    //            $assoc['type'] == ClassMetadata::MANY_TO_MANY &&
    //            $val instanceof PersistentCollection &&
    //            $val->isDirty()) {
    //            $entityChangeSets[$oid]   = array();
    //            $originalEntityData = $actualData;
    //            $uow->entityUpdates[$oid]      = $entity;
    //        }
    //    }
    //}
    /*********************/
    return $entityChangeSets[$oid];
}
It's phrased here as a static method but could become a method inside UnitOfWork...?
I'm not up to speed on all the internals of Doctrine, so might have missed something that has a side effect or misunderstood part of what this method does, but a (very) quick test of it seems to give me the results I expect to see.
I hope this helps somebody!