Question for you guys and gals:
I do most of my composing and performing on my own, and sing my own songs for pitching to labels. When I do work with others, I collab in the creative process, so I don't know much about how this work but:
I've got a songwriter who wants me to sing on a demo he's pitching to major label country artists (not collaborate in the creative process, just strictly singing), then mix and master. I'm asking for a flat fee, but what would happen should the song get picked up by a major artist? I know the singer on the demo isn't entitled to any "royalties" since he did not participate in the writing of the song. But can the singer ask for some sort of compensation/kick-back/bonus should the song make it big? I mean, after all, the vox helped sell the song!

I know there's lots of collaborators, session players, etc. in this community, so how would this typically go down?
Thanks in advance....
-Yaps