The fastest way I found to get this implemented was to combine the answer from Chelmertz with this thread.
Instead of defining a decorator to show the previously uploaded file, I am placing the link in the description for the element and then I am dumping it into a view script.
The final solution looks like this:
Zend Form File:
class Application_Form_LabDetails extends Zend_Form{
private $_uploadPath;
private $_artistID;
private $_singleFile;
public function __construct($options = array()) {
    if(isset($options['uploadPath'])) {
        $this->_uploadPath = $options['uploadPath'];
        unset($options['uploadPath']);
    }
    if(isset($options['artistID'])) {
        $this->_artistID = sprintf("%06s",(string)$options['artistID']);
        unset($options['artistID']);
    }
    if(isset($options['singleFile'])) {
        $this->_singleID = $options['singleFile'];
        unset($options['singleFile']);
    }
    return parent::__construct($options);
}
public function init()
{
    $this->setName('artistDetails');
    ...
    $singleID = $this->createElement('file', '$singleFile');
    $singleID->setLabel('Current Single')
        ->setDestination('data/uploads')
        ->addValidator('count',true, 1)
        ->addValidator('Size', true, 5242880)
        ->addValidator('Extension', true, 'mp3');
    if($this->_cocFile){
        $singleID->setDescription(
            '<a href="/'.$this->_uploadPath.'/'.$this->_artistID
            .$this->_singleFile.'" taget="_blank">'
            .$this->_singleFile.'</a>')
        ->setDecorators(
            array('File',
            array('ViewScript',
            array('viewScript' => 'artist/file.phtml', 'placement' => false)
            )));
    }
    ...
}}
View script to handle file details and form element:
<!-- decorator through view script,  placed in 'views/scripts/controller/file.phtml' -->
<!-- outputs form label markup -->
<dt id="<?php echo $this->element->getName(); ?>-label">
    <label for="<?php echo $this->element->getName(); ?>" 
        class="<?php if ($this->element->isRequired()): ?>required<?php endif; ?>">
        <?php echo $this->element->getLabel(); ?></label><br />
    <span>Uploaded: <?php echo $this->element->getDescription(); ?></span>
</dt>
<!-- outputs form element markup -->
<dd id="<?php echo $this->element->getName(); ?>-element">
    <?php echo $this->content; ?>
    <?php if ($this->element->hasErrors()): ?>
        <ul class="error">
            <?php echo $this->formErrors($this->element->getMessages()); ?>
        </ul>
    <?php endif; ?>
</dd>
Lastly in the controller I have:
//$id is taken from the URI
if ($id > 0) {
    $artistDetails = new Application_Model_DbTable_ArtistDetails();
    $artistData = $artistDetails->getArtistDetails($id);
    $options = array(
        'uploadPath' => $config->uploads->labs->path,
        'artistID' => $artistData['a_id'],
        'singleFile' => $artistData['a_singleFile'],
        );
    $form = new Application_Form_LabDetails($options);
    ...
}
This should give you something that looks like:
