I guess I don't understand your question...but here are some generalizations, that might help answer whatever it is you are trying to figure out.
No you can't have a postban Assualt Pistol in MA (because you want to make an SBR out of it doesn't change that)
Keep in mind the MA definition of FIREARM, if you find an FFL who understands it as well you could buy a stripped barreled reciever, and go from there...
The "upper" of HK style guns is the reciever, jump on gunbroker, there are ussually a few for sale, of course it will need to go to an FFL. unless of course its been demilled, then its just parts, so if you want one of the cut in half MP5's you can just buy it and go from there.