I doubt anyone in Ma. has the PredatOBR however I may be wrong. I shoot a Larue and I've yet to see another one in the state even though I know they exist.
You might want to check here:
http://www.ar15.com/forums/f_2/219_LaRue_Tactical.html
if you haven't already as they would have all the info on the PredatOBR. The barrel nut is deigned to be QD so I would imagine it's fairly easy to remove/install, their are plenty of videos on that forum that show assembly/disassembly of the TOBR and it looks real smooth.
You would have to buy the AWB compliant version which in any case (regardless of barrel length) would come with a muzzle brake welded in place. They only sell it as a complete rifle so you have no choice on the matter unless you move.
One thing you might want to check beforehand is that you get a muzzle brake skinny enough for the barrel nut to fit over so you can completely remove it/switch it to another barrel if need be.