I recently bought a Redline Monocog that has the exact same problem. Tighten the crap out of the top cap (get it tight enough that the bearings wouldn't turn smoothly) and the headset still clunks when the front brake is applied.

After reading much about what junk the stock Monocog HS is, I took it back to the bike shop, and he contacted Redline. The sales guy said he knew what the problem was, and that it was a warrenty repair. He is shipping the parts to fix it to my LBS, and they should be arriving any day now. The LBS owner didn't know what exactly the parts consisted of, but I bet it's the same problem that you're having, and whatever parts this mystery bag contains will be the solution to this puzzle. I'll post here when I find out more.