We're looking to include a iOS sticker extension in our latest game. It'd have to be a iMessage extension app however as we need dynamic content to be displayed.
Basically, it'd act as a regular sticker pack however the player can unlock extra stickers inside the corona app. We could save data as a json/xml file from the corona app and then the iMessage extension app would read that to determine which stickers to display.
This thread kinda explains what we want, however we wouldn't be charging IAP for stickers, but rather rewarding players for completing certain in-game tasks:
If you're interested, please let us know how long you think this would take and how much you'd charge.