This is a tricky one - you have to approach it carefully or it could seriously backfire. I know it's a bit daunting, but just have a word with her and see what she says. The worst that could happen is that she says no, and in that case you haven't lost anything anyway.
Maybe try to plan it out, so she knows when to expect it. My ex boyfriend once tried to have sex with me while I was sleeping without asking first, I woke up to find him doing it and I felt violated. I explained to him that I wouldn't have minded if he had asked my permission first, but it's not okay to just do it unexpectedly. He didn't really understand this - one of the many reasons he's now my ex!
On the other hand, I regularly touch my OH and go down on him when he's asleep, although I know he really enjoys waking up to a surprise so it's not a problem. :)
To summarise, there's nothing wrong with what you want to do at all, and I can understand why it's a turn on (I think I'd enjoy it with my current partner), just make sure you talk it through first :)