For use on "post-ban" barrels with poor, homely plain muzzles.

Why not just have the barrel threaded?  Its cheap and allows the use of any flash hider you want.

Because doing so requires one to remove the barrel from the receiver and send it off in the mail?  I would think that a bolt on FH would work unlike a muzzle brake due to the fact that there is not a lot of force being applied to the FH in the forward direction by the muzzle blast.

As for a answer to the original question, so far I have not seen anyone offer one, but I figure someone will eventually.

You can send the whole upper and have it back in a week.

I thought about what your trying to do.  I gave in and sent it off to get it cut & threaded.  It's not going to take long, and in the long run you'll be glad you did it.  Especially if you ever want to put your upper on the open market.  It won't draw near as much $$ as it would if it was threaded.  Just try not to add to the list of things being done to yours like I did mine.  I started out with just getting it threaded and ended up asking for more.  

CMMG doesn't require you to remove the barrel from the upper.  I'm not sure about ADCO, but I've heard that they have a fast turn around time.  They may want the barrel removed.  I'm sure someone will come along that has had it done with them and let you know how they work.

Good luck