As attractive an image as that presents, I'm almost certain that Sarah would have nixed the idea. The closest that she ever came to nudity on the show was an occasional bare shoulder. I honestly don't think that she was comfortable with it at that time, though she has recently, for whatever reason, relented and said that she would "consider a topless scene if it was required for a particular role." And as far as Spike is concerned, a naked Buffy would have done just the opposite of disorienting him. It would have gotten him fully focused and totally motivated.
Plus, of course, there is the fact that Buffy was aired in what is considered "family time," and the censors would almost certainly not allowed a scene like that to be used. Granted, James had on several occasions appeared wearing only his famous sock, and Angel had popped back from hell wearing nothing but dirt and blood, but there was never anything more than a brief flash of skin, whereas Buffy/First Evil would have had to remain nude and visible for more than just a few seconds at a time to be convincing.
Day 18. 12 days to go.