I think you have to smooth talk your way into this. Casually toss out the fact that you used to really like and miss making out with her. Make sure you do this at a good time and with good delivery (not too serious.) Depending on how she reacts to that, at a later time (maybe after a drink or two to loosen her up) tell her how much you really wish you could go down on her. Reassure her, immediately, with something like, "I know we're just friends, and i wouldn't want to jeopardize that, but i really would give anything to kiss you there right now. I know this might sound sudden, but i don't expect anything back from you, and i'll stop anytime you like."
Maybe that'd work. If not, well ... best of luck!