Basically, to try to answer your question, as I understand it there isn't a way to do this. As you would be presumably having to manually search for a given cache anyway (I'm not sure how else you would pull up the details for the specific cache), it would be almost as easy just to do a manual search on geocaching.com and look who got the FTF.
Even if there were a way to do it, that would only work if they'd logged in a way which is recognised by PGC, as you've said. If they hadn't, you'd still have to go look at it manually anyway.