fubsy, I know that "Barrel Compensators" for un-threaded barrels are available for post-ban AR 15"s. They attach with four Allen head setscrews. I don't think the Vortex is available in this un-threaded configuration. (If you can find one, find two and I'll take one.)
I don't believe that the BATF will approve the addition of the Vortex, in the threaded configuration, even if it is permanently attached to the firearm. I believe your local BATF Inspector, as George suggested, will be your best bet for the final word.
Please let us know how this shakes out for you. I'm sure I'm not the only person curious about this.