No pistols are legal or not legal....
Not to nitpick, but that's not 100% accurate. There are indeed pistols that don't comply the AWB and therefore are technically illegal to posses in MA.
As you mentioned a pre-ban would be one way to go.
PROS: No silly AWB restrictions on weight, barrel shrouds, etc
CONS: Expensive, limited availability, some potential legal gray area
Another way would be to build one from parts that complies with the AWB once assembled.
PROS: Cheaper in theory, though buying specialized low-weight parts definitely adds up quickly.
CONS: Silly AWB restrictions basically make the end result pretty much useless.
I looked into doing this at one point (even bought a polymer lower) and although theoretically possible the hassle and expense were just not worth it to me personally. I agree that SBR or AOW are more practical and certainly easier to do (fewer hoops to jump through.)
Simple answer to the OP's specific question: No, the Bushmaster pistols that are currently on the market are not legal to posses or transfer in MA unless you happen to find one that qualifies as pre-ban. (And I'm not sure that pre-ban versions even exist. I ~think~ those came out after the ban.)