I like the idea. Someone around here in Canada was running the same service, I only know if it vaguely. Add in some bulletproof evidence, like photos and whatnot (possibly of a girl holding up a sign which can be customized with the client's name) and you've got a winner. It's not depraved - if people are willing to do it, it's not like you're killing babies. Just a harmless little white lie.
But then, if they stop paying, threaten to anonymously inform the client's parents that you're pregnant with the his child and the loser deadbeat won't even admit that it. Like they say in Training Day: it's not what you know, it's what you can prove.