If you feel the need to follow the "NOT law,", build your lower as a 22LR and EFA10. Then you can slap whatever you want on it later (which does not trigger a requirement to FA10 again).To note, it was my understanding that lowers acquired before 7/20/2016 in the state of MA were legal to build because they were transferred into the state. As stripped lowers, they are not able to be registered until built, but if they were in fact transferred to someone in state before 7/20/16, you can still build and register.
I would assume the best thing to do is have a receipt showing the sale and transfer date since theres no FA10 record on the lower when purchased since it is not a "firearm". Granted, if they really want to take the time to look into it and check the 4473s, they have enough to temporarily break your balls. I know I would feel better at least having some documentation on the lower to avoid the hassle of bringing a legal rep into the matter while they investigate it.
But maybe I'm not the best to advise, because I believe we are on a post-law world on this country now.