You may be right with that.How to fix iMessage by calling to Apple has already been mentioned a couple of times, also by me just a couple of pages back. You don't have to give them a fake SN, your old unsupported SN is just fine.
I knew that I had to call them but I wasn't quite sure what to tell them. And that was my actual question as I hadn't seen your postings before.
I only saw a couple of other users who said that the iMessage fix is just a phone call away, but I was wondering whether the Apple service would notice that a 2006 Mac and iMessage don't belong together, since the serial number indicates the exact model and I would never have thought that Apple gives support for an 'unofficial' system.
Anyway. In your case, good to know that it worked that way.