Well Colt had done this in the PAST as about 3-years ago there was a member of this forum that had sent a dammaged Colt SP-1 lower and they replaced it with a BRAND NEW Colt A2 lower complete with mil-spec push pins, mil-spec trigger components and NO SEAR BLOCK! Hell, I ALMOST considered attacking my Colt Sporter with a hammer for that deal!
Good Luck, but I know for 100% certanty that they had replaced lowers in the past. There was a fee and I don't remember how much, but I do remember it being well worth it for the end result.... Now, I didn't know that Bushmaster would do it also!
Oh yea, the lower that was returned really was dammaged by legit means. As I recall my old friend from the forum had received it in trade and it was basically unusable in the condition he got it. He took the chance and made out like a rat (RATATAT at that)!