my back was pretty low as well, i had a full strapless bra, and i was concerened as well! the seamstress at davids bridal left the top 3 hooks un-hooked and she folded them under so you couldn't see it, it was a genious idea i didn't think of!
i have a pair of those sticky "nu bra" things that stick straight to your skin and you can re use them over and over. they retail for like 65 but I got mine for 35 at steinmart, and they are WONDERFUL, do not come unstuck at all and very comfortable, I wear them all teh time instead of a strapless bra that I have to keep pulling up! *haha i'm actually wearing them in the pic. in my siggy*
I had this same problem. I wore a backless/strapless bra that just like a regular underwire bra on the front, just it has stickies on the sides. It worked great! I was worried that the humidity would not let it stick, but the girl at the store told me that the humidity would make it stick better and she was right! I found a link for the exact one I have. They give you 3 sets of stickies with the bra, and you can even use a set to have the bra also stick to the dress (incase you worry about it staying up). I just used the stickies on one side, but it is a good option.