Brownells sells two AR15 butstock with thumb holes (thus eliminating the pistol grip).
If the pistol grip is removed (on less evil under post-ban laws), may I add a pre-ban Flash Hider/Suppressor (A2, Vortex etc...) and still be legal under post-ban laws? If so, must the pre-ban flash hider would be permanently affixed? Of course no bayonet lug.
Thanks

No - the thumbhole stock still counts as a pistol grip under the 94 assault weapon ban, so you can't add a feature in its place.

There's a little teensy hook-looking thing that can be installed in place of an AR pistol grip if you wanted to eliminate that, and you can then add a flash hider - but not a standard A2 or vortex, because they're considered a grenade launchers, too.  You'd need to find or have made a flash hider smaller or larger in diameter than NATO standard 22mm muzzle devices.

The flash hider need not be permanently affixed, if it's your one evil feature.