I, unfortunately, have sent spam messages via iMessage a long time ago. I did it by faking a Bluetooth keyboard that connects to an iPhone and starts typing out phone numbers and addresses fed from a computer. I did it to mass-send a message to about a couple hundred people.
Desktop macs have i message too, so a determined spammer could just make a bunch of mac vms. Getting a valid serial with an unconfirmed purchase date is a massive pain, though.
