I suppose you could order the rifle at a free state shop with a gunsmith, have them trim the bayonet lug, swap or pin the stock and permanently attach a brake. Federal transfer to you, then FA-10 as a registration when you get back in MA.
Unless no bayonets, collapsing stocks or flash hiders was a deal breaker, of course.
Option B would be piecing together the same features, which may, or may not work as well as modifying the 6920. Tough getting some of those parts these days.