Hey @jdr,
That's not too surprising, since it can't be done if the given application doesn't have AppleScript support.
Unless of course you know the magic spell.
See this:
Finding the File Path for the Front Document in the Front Application
If you can't figure it out from there then holler at me.
@peternlewis – this comes up with some frequency – would it be possible to add a token for the path to the front document in the front application?
-Chris