Going through the same dilema, Spectre. I bought a Bushy 20" uppwr with an AK brake on it and in makes the overall length of the rifle 40.25 inches. Just a little big for a 40 inch discreet case (obviously). I was thinking of sending it off to Kurts Kustoms for either his A2 Brake or Fake Flash Supressor. either that or look for a 40+ case. I sent him an M4 upper with a full 16" barrel and a mini Y comp tacked on to that. It just looked too long. I decided on having a FFS telescoped over the end (ala LEPG M4). My reasoning was to keep the full 16 inches, but now I'm having second thoughts about not havin a brake. I started doing some CBQ/Entry stuff and my origional AR (AK shorty now with an LE Entry stock) is perfect for the job. Next time I'm going to try it with my M4gery and see how the lack of having a brake is. I don't think a 20 inch HBar would really need a brake, but that's a descision I'll have to make down the line.
In eather case, have it tacked on to the end and not telescoped, unless you want the Dissapator look. Having it tacked on to the 20" barrel will still be under 40 inches and have the look of a preban 20" with a Flash Supressor. Just check out the Bushmaster catalog under Law Enforcement models to get an idea of what it will look like.