Valve, please do something. Fixing the anticheat wouldn't only fix tf2, it would save many other multiplayer games plagued by cheating, including other Valve games like CS and the upcoming Deadlock.
#SaveTF2 #FixTF2
Any client rejection would have to occur server side. I'm assuming that the client doesn't even report to the server if it's running headless. Nonetheless, it'd be trivial to mod the client to report that it's not running headless even if it is.