A friend of mine was in a similar situation so this is what he tried.
He put his name and number on a piece of paper went over to her and said something like this:
"Hi/hello, i can't stop at the moment but i would really like to buy u a drink/coffee/lip stick?? here's my name and number send me a txt if u fancy meeting up."
There was more said than that but i can't remember it at the moment, i'm sure with a few more good lines u could be onto a winner.
not a very good explanation i know but the girl in question was very flattered and called him later on that day. so it did work.
Now i've just typed this it seems quite lame but i'll post it anyway :D