Emulating iMessage is an awful job that only a select few seem to have managed. The protocol is confused and complicated, the binaries are heavily obfuscated, and Apple's servers ban quickly and with little warning. Even those that have succeeded don't seem to have emulated it correctly.
Yea, but that's if you want to emulate iMessage to the Apple servers in order to send actual iMessages. I think the OP was asking about emulating it to the airplane firewall in order to tunnel through, which could be a lot easier and more useful.
It's unlikely that they're inspecting the packets at a level deep enough to tell what they actually are. More likely that they're just letting you access certain servers for $2.