Here's a clip of various metal/rock stars in movie cameos. It's a little long (almost 10 minutes), but GnR is in the first minute or 2. Worth watching till the end to see Steve Vai tearing it up against the Karate Kid.
https://www.youtube.com/watch?v=dY2NoXkaLdU
https://www.youtube.com/watch?v=feHmiwsv86k&feature=related
PS the term "cameo" is pretty loosely defined here. Dee Snider's role in Strangeland was hardly a cameo, but you get the idea.
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I used to be with it, but then they changed what "it" was. Now what I'm with isn't it, and what's it seems weird and scary to me, and it'll happen to you, too.