She asked for some time to digest what I was asking of her. I agreed. We settled on a tentative date of one month for her to give me an answer, with periodic check-ins between now and then for her to ask me any questions that occur to her (she already asked me several intelligent questions about how I planned to pay for it, what kind of timetable I was looking at, etc.) and for her to let me know where her thinking is at.
A couple of people have pointed out to me that I should be very wary of believing any promises that she makes. That she might deceive herself into believing that she could "grow into it," or try to get me to put the decision off for a few years in hopes that I'll change my mind. I think the danger of that is lower in her case than in most cases: one of the things I love about her (and that infuriates me about her) is her uncommon stubbornness and unwillingness to change or compromise the core of who she is for me or anyone. So it's a lesser danger than it might be for another person, but it IS a danger.
I really think that ultimately she will come to the conclusion that this isn't something we are going to be able to find a middle ground on, and that will be the end of our relationship. But I owe it to her to give her the time and space to come to that conclusion herself. If she doesn't... well, I'll cross that bridge when I come to it. I haven't ruled out the possibility of having to tell her, "I'm sorry, I'm deeply touched that you're willing to make this compromise for me, but if that's what it is to you, a compromise, then this isn't going to work."
To me, this has stopped being about getting her on board with what I want from this relationship, and become about ending it in the right way. I really think that we have a chance to do that now. I told her that I wasn't trying to bully or blackmail or even influence her, that my highest priority wasn't her agreement or obedience but an honest answer. I told her that I still love her, and even if this means that the nature of our relationship has to change, I'll never stop loving her.