Are there other scores, besides BATMAN: MASK OF THE PHANTASM where Hans Zimmer has not contributed to the score, but is just invited on to perform a instrument (e.g. synth)
Thor, in case you don't get it, - it's film that feature Hans Zimmer in the score but HE HAS NOT written the score (Pirates of the caribbean does not count since he actually wrote that one)
He's often thanked in the liner notes of various releases, but I assume that has more to do with some producer help, borrowing equipment from RC etc.
In terms of "guest performances", I'm not sure. There's obviously the non-film stuff (like The Buggles, his recent collaboration with Jean Michel Jarre etc.), but that's not what you're asking for, I guess.
Then there are MANY soundtracks where he has just contributed a theme or two, but not written the bulk of the score. Does that count?