I've always avoided using mac pro 6,X SMBIOS ... for all my desktop builds i use a iMac SMBIOS which use desktop CPU's rather than enterprise .... Intel Xeon CPU's contain
Vpro architecture which has an additional layer of security embedded in the CPU itself (kind of like SMUUID but specific to the CPU rather than the motherboard). its possible that Apple use some of that embedded security in their validation on the Trash-Can mac's ... it's just a theory but a sound one ... I don't think there is any reason to use 6.X so try a iMac one ... I've set up countless hacks using iMac SMBIOS on all sorts of PC hardware and never had an issue with iMessage/FT.
You could try posting in one of the build threads and ask if anyone using Macpro 6,X has had any luck with IM .. but unless your using a Xeon CPU .. it might be a dead end ....
IM/FT works across all my hacks (laptops & pc's) and across all my apple devices (iPhones, iPads .. etc) see the Reset IOS networking tip at the end of
Chapter 3.3 .. i haven't had use it in a long time but it might help.
Jay