I noticed that Bushmaster sold their dissipator barrels with a bayonet lug. How can you put a bayonet on it if the barrel isn't long enough? Tell me if I am being stupid.

No, you are not being stupid (unless you're doing something else that IS stupid, of course), the Dissipator will not mount a bayo, despite the lug. I have a tac light that I think will mount on a Dissipator (it was used on a short barrelled sub-gun). The barrel ring and the bayo latch are very close together. Similarly, a 16" barrel has problems mounting a bayo since the barrel ring on the bayonet rides too far back and is on the barrel instead of the flash suppressor.