Far2 - I suppose anything is possible. Going with that premise and building on the feelings and experiences we've had, especially recently, I think I would be willing to try most anything she truly wanted. I think that's what's made it both easier and more enjoyable for me despite the obvious implications - it's been knowing that it's what she wanted.
I do think the writing is on the wall - that for at least a little while - if she found someone who filled the role she is looking for - that she would want me to, at some point, use condoms with her. From our conversations we've had - I expect that at some point she will want to experience time with her lover as she did for her own self with Frank. Even though it isn't something I would wish for, I know she will want it and I already know that I will acquiesce.
You are asking beyond just condoms, if I'd ever agree to stop having intercourse with her completely. I can't see that right now, stopping totally, but in the spirit of your question, if it was something she absolutely wanted to try, again, I think it's obvious that however reluctant I am, that I would be okay to try it out - but again, I don't think it's something I could accept as a norm - I don't believe I'd be comfortable only being relegated to having sex with her just on special occasions.
Funny though, as I'm writing all of this, the thought that "well, if I got to have her bare on those days....." did go through my head. But isn't that expected if I enjoy being a cuck - so I suppose, in the infinite world of a million monkeys typing a million pages, that perhaps in the right circumstances, it might work.
Really the biggest factor is that Sue would genuinely want it.