I don't know if serials matter. I'm using unrecognised MacPro serial right now and it works. It used to work with an Mac mini serial before that (never checked the Mac mini serial number).
I don't know which particular thing got iMessage working, but I did do the following sometimes in the past:
- added real credit card to account
- registered free developer account to get the beta updates
All the numbers in the macPro SMBIOS right now are generated by CloverConfigurator. I didn't do anything special, besides fixing the ethernet kext issue.