Terry Melcher lead on demo for Happy Endings (idk if you consider that one)
Then there's John Phillips' early renditions of Somewhere Near Japan and Kokomo. (That may not be what you're looking for, but that's what I thought of)
I guess those are grey areas, but probably don't really count, seeing as I'm not sure what those songs (SNJ + Kokomo) were written for originally - I assume as potential John Philips solo material. Plus then we'd have to count Cease to Exist. I'm more thinking of instances where we have a partial backing track that eventually gets used for a BB release, but we have a non BB singing it as a demo for some reason.
A couple more things to ponder...
- Was the Rocky Pamplin-sung California Feelin' vocal recorded as a scratch demo for the BBs, or as a demo for a potential solo project of his?
- And I assume that random Amy gal singing along to the backing track of Let the Wind Blow was done just as a test to hear her vocal chops after the full BB version was already released or in the can?