Whether this is a bug or a technical limitation I do now know.
Maybe worth reporting an issue if there is none yet.
It seems the class loader from the init script is not in the upwards hierarchy of the class loader from the build script.
The releasePaperwork extension is seen, so an accessor is generated for it and it does not complain about that one.
But its type is not found and so is of type Any which does not have the tagPattern property.
If you try to cast this, the extension class to cast to is not found.
If you simply add the plugin to the classpath of the build script too, even without an explicit cast, you get a cast exception.
As long as you do not add the plugin to the buildscript classpath you can work-around the problem using: