The gold composit lining inside the canopy glass itself should be more than enough to shield all the internals, including the brace, from incoming enemy radar.
As for the HMD sensors compromising stealth, well for one, all existing head tracking technology used in HMD/HMS are optical based (sensors look for the positional change in all those little tracking balls built into the helmet), so there should be zero EM emissions. Even if they opted to used some sort if EM based sensor for the HMD on the J20, I think they would have bigger fish to fry, if you excuse the pun, if the emissions are strong enough to be detected by enemy sensors are anything like meaningful ranges, after being mostly blocked by the same gold composit tainting, as such a strong EM field within the cockpit will likely lightly microwave the pilot...