I already have my pom.xml set up to do some basic replacing on a file during the prepare-package phase:
<plugin>
    <groupId>com.google.code.maven-replacer-plugin</groupId>
    <artifactId>maven-replacer-plugin</artifactId>
    <version>1.3.8</version>
    <executions>
        <execution>
            <phase>prepare-package</phase>
            <goals>
                <goal>replace</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <file>target/classes/plugin.yml</file>
        <replacements>
            <replacement>
                <token>maven-version-number</token>
                <value>${project.version}-${project.build.number}</value>
            </replacement>
        </replacements>
    </configuration>
</plugin>
However, I'd like to have that value be conditional on whether a particular environment variable is set. I run builds through Jenkins, and so I'd like to check if the environment variable BUILD_NUMBER is set; if so, it should take the place of project.build.number.
Is there any convenient way to do that within the replacer plugin? I read over the usage guide, and couldn't find anything immediately useful.