That Mossberg was not using a Knoxx Stock, I think they use the ATI stocks on their tactical guns. The Knoxx is well-known in the shotgun community, and I have never heard of one failing like the Mossberg did. They are specifacally made for heavy recoiling shotguns. You can read alot about them at shotgunworld.com or ar15.com in the Arm & Tactics section under shotgun.
The trick would be getting one to fit an NEF receiver. An adapter from the reciever to the AR tube would be fairly easy for a machinist, but I don't know how you would address the grip being back too far from the trigger.
Jon