I hadn't even thought about the possible implication of an ulterior motive! Good point.
After pondering all this excellent advice, I do prefer the idea of separating the transactions. Complicating the process by mixing in a completely different element at this point wouldn't be a good idea, IMO; many of my clients are confused by how portrait commissions work in the first place and need a lot of reassurance! Once they're more comfortable with that and with me would certainly be a better time to bring up possibly dicey issues like my proposal.
Since I've got shoots on Monday and Tuesday, I'm going to spend some time this weekend perfecting my model release form and adding in the element of possibly using the photos for other paintings. If I'd had the presence of mind to have the client sign something at the shoot in the first place, I wouldn't have opened quite so large a can of worms for myself.
So I'll deal with them one by one!
Laura