Permanently attaching a muzzle brake to a barrel will make it "legal" in the sense that you don't need to SBR it, if attaching the muzzle device brings the length of the barrel up to 16". Just be aware that if you are doing this on an AR and taking measurements, you do not include the barrel extension in the length of the barrel. It is not permanently attached, in the legal sense. In other words, pinning a muzzle device to a 7.5" barrel isn't going to make it legal unless it's a really long muzzle device.
On the other hand, if you are building from a virgin lower receiver, just build it as a pistol and use a brace instead of a stock and not worry about the length of the barrel.