Would have responded a month ago if I caught this post... Absoposilutely it would count as a US part if you made it from scratch in the US.
However, you'd be responsible to be sure that it only works as a muzzle brake, not a flash hider. Many muzzle brakes and brake designs also work as flash hiders. When someone advertises a "
BATF approved" brake, it means they went to the trouble to have
BATF confirm that it is acceptable as a brake, not a hider.
Of course, some things may be obvious in design, so you may know beforehand how it would work.
Thinkin' about makin a muzzle brake 
