Yes, I've painted myself into a corner.
I definitely take the point about the separate model release, and I'll start using one immediately. I appreciate the advice very much!
There's a clause in my boilerplate contract that assigns copyright in the photos and portrait to me, and another clause that specifies that I can use those photos and portrait for self-promotion and so on, "in all forms and media and in all manners," etc. I'm not dead positive that stretches to include other projects, so I do want to make certain about that, and I'll put that language in the model release.
This is a slightly unusual situation as far as payment and contracts are concerned. I donated the commission to a school fundraising auction. So the client has already written a check, albeit to the school and not to me. At the time of the auction, she received a spec sheet that I had already signed, giving the exact terms of the commission: 9x12, pastel, one subject, head and shoulders only, etc. That's the initial contract.
Unless she decides to upgrade her commission to a larger size or a half-length view or something on that order, she won't owe me any money. The way I've usually proceeded with these donated commissions is to do the photo shoot and show the client several pose options as an incentive to upgrade. It's pretty common that they end up wanting to do so, even though my major reason for the donations is self-promotion as well as building my portfolio. I'm a beginner--can you tell?
I didn't think I could ask them for anything up-front before they've made that upgrade decision, so I dropped the ball on the model releases. When it's an ordinary commission from scratch, I do ask for a deposit at the time of the photo shoot.
Covering my rear isn't the only reason for all this, of course--I don't want to cause offense. I dread a situation where the client or a friend of hers spots something that I painted from photos of that child and raises a stink. That's why I say I don't want to sneak anything past her. If it ends up that I just can't use those particular photos, I'll grab my kids, take them out to the park and fill up a couple of 512 meg cards!