One problem I see is that you COULD detect when someone taps a particular sticker and readies it to send, but I don't think you can actually test to see if it got sent. So if the user taps it and then decides, "nevermind", you might end up deducting them unnecessarily. But then, hey, maybe the users learn they shouldn't initiate sending a sticker.
On the recieving end, you're assuming the person getting the sticker already has the app installed right? Because if not, that functionality you describe isn't going to work obviously.
Plus I haven't come across anything that identifies a particular sticker when recieved (maybe that does exist though). You might have to do that trickiness by sending an MSMessage not MSSticker. In which case you can pass around URL variables that would identify what's being sent. But that could work, so for example, your button to send to a friend might simply say "Give a friend 20 Dog Stickers", so its not actually sending a sticker but a message that person gets which unlocks the stickers.
Anyway, my two cents. I've taught some iMessage lessons by the way, so I do think you're in the realm of possibilities.