I called Bushmaster and they told me that due to the risk of damaging the barrel crown, they are not going to be threading barrels. This sucks but that is what they told me over the telephone a couple days ago.
Speaking as an ex-machinist their concern about damaging the muzzle crown doesn't make much sense unless they think they'd need to run a live center in it. I assumed that the barrel could be chucked close enough to the muzzle that deflection wouldn't be a concern.
There's a guy here in Houston who'll do them for $35 if stripped and $55 if he has to dismantle the upper. I'll ask him about his procedure.