I'm curious about that, since iMessage and iTunes Match have been a constant source of trouble to one of my colleagues. He also mentions that the device is flagged as "Untrusted" in Find My Mac...
The idea is to generate a valid serial number (in form) that is not used by someone else. Not sure if things have changed, though, but I used this last year and it worked. It still shows up on my account as an iMac 2013.
https://www.reddit.com/r/hackintosh/comments/2wohwn/getting_...