To the OP- I sympathize with your situation, most of these cold posters have no idea of the struggle that goes along with an applying significant other. It is no surprise to me that these schools value and give preference to your relationship. It shows devotion and commitment, qualities obviously needed in life as a physician.
My 3 yr boyfriend got into my dream school, and I'm headed to a place 13 hrs away, somewhere I definitely didn't imagine myself initially. Boy what stress this application process has caused. Wish I could "bargain" my way in, but I don't think that would work (he's on rough grounds right now). At this point, its about turning lemons into lemonade.
So I'm here only to say what I would do if I were in your situation; it's not necessarily what you have to or even should do. I would go ahead and do the same significant other email protocol on UCLA, it can't hurt. Clearly, you cannot predict your UCLA acceptance or her USC waitlist. But you can tempt the hands of fate with all of the powers you DO have. This is how I would "bargain"
Each of you can send letters to your existing acceptances and say something along the lines of, "I'm really excited about coming here, etc, I'm struggling with deciding to come here and going ___, the biggest factor is my gf/bf, who interviewed here. While I love this school, my relationship is important to me, and this is going to influence my decision, etc. Obviously I have a big decision to make, to make this struggle any easier I was wondering if you had any advice for me...blah blah...Additionally, what kind of support/special arrangements(?) are available for committed couples with this college...Any advice would be appreciated."
Here's the way I look at it; YOU'RE ALREADY IN. It really doesn't matter what you say, they can't de-accept you on these grounds. Who knows what would come of it, but at least you can feel assured that you did everything you could.
Best of Luck...