Clinton, my opinion is that "mouth" is a much better word here, for a number of reasons--not the least of which is that the kind of kiss he's probably talking about involves much more than the lips...! By the way, I believe "scan" is generally used to refer how stressed and unstressed syllables are arranged in a line of poetry--so in that sense, both words "scan". You're probably talking about the sound of the word within the line, and I think I understand what you mean. But I suspect image and meaning won out over sound in this case. For me, at least, "mouth" makes the line much more powerful. In fact, maybe that's in part because it doesn't "fit" into the line quite as well, and so the image becomes even stronger. I wonder if "lips" might not have been in the first draft, though. Fascinating.

Aloha,
Mark