It sounds like you jammed yourself up by not knowing that there are no prohibitions on stripped lowers in MA.
Just buy one and stop worrying.
What matters is how you build it. It must be built in compliance with MA AWB. But other than that, lowers are no problem.
With respect to 80% lowers, they are pointless unless you want the satisfaction of knowing that you built it.
If you want a lower without MA knowing about it, just go to NH and buy a lower from a dealer. Perfectly legal, and you don't have to register it in MA until you actually make it into a functional firearm. Which you'd also have to do with an 80% lower if you wanted to stay legal. Buying a lower in NH would require the dealer to submit an ATF4473, which means the feds would know about it. But the feds are the least of your worries in MA.
Also, if you didn't care about being legal, you could go to NH and buy a lower from someone face to face. I'm certainly not advocating this. I'm just anticipating a response that says that you should buy an 80% lower and then build a gun with it that "nobody will know about". You can achieve the same thing by buying from a gun friendly state from a private individual. Both result in an illegal gun. But one doesn't require you to machine or drill anything.
So in summary 80% lower = pointless.