If someone's first line to me in an elevator at 4am is "do you want to take me up the bum?" or something similar, it's just.. well, you've got to work up to that, you can't just ask it right away. "Hi, my name is Susan, how are you, yadda yadda, would you take me up the bum?" - now we're getting somewhere.
Not to make light of the situation, but it's incredibly context specific. There's a creepy way to proposition someone.. and while it's not easy at all to make a non-creepy sexual proposition in an elevator at 4am, it's not entirely outside of the realm of possibility. Just be a gentleman/woman, don't be forceful, smile, introduce yourself, and you might very well come off as someone the other person might want to spend some quality time with.
Granted, most people would come off as creepy in that situation, no doubt.