I can't speak for anybody but myself Doug, and while I don't shoot women for money, I'm definitely a whore, mercinary, schill, whatever... but a promise is a promise, even on a contract it's a personal agreement you make anew with each session. I routinely agree to never release images of products... of course it's not quite the same thing though, is it?
You don't have to make that absolute promise every time. You might be able to persuade someone to let you make an exception after the fact. That's all fair to me as long as you stand by what you agreed upon. Your integrity is unbroken, and like you say, you get shoots that would otherwise be unattainable.
I know what you're feeling about wanting to share a special image every now and then. Even with my limited experience shooting just one woman I have images I can never share that are some of the best photographs I've ever made... and I wouldn't dare post them because I want to keep shooting her in that confident, uninhibited way that ultimate privacy yields.
I have however, persuaded her to let me share some images that were off limits at first... and that too, has yielded a level of confidence in her posing that was previously absent.
I don't think you have to resort to grey areas, I think you can negotiate with your models and perhaps get permission to share, or even hang some of those transcendental shots that crop up. As long as no always means no, then you're golden, your integrity is solid and worst case, you can just smile knowingly with your tongue in your cheek.
Beth says I can show this one...
I haven't retouched it yet, just shot it last night... exactly one month after her 43rd birthday.