Keeping each flight anonymous as possible means it will be difficult to pick off a particular General or other leader, but it's a target rich environment and picking off even one, with no one high ranking aboard is enough to send a message. While I get that being sneaky lowers the odds, they are not reduced to zero as long as there is a common hub and a few narrow courses. Like, if someone managed to smuggle an anti-helicopter missile into the area, they wouldn't need ADS-B to tell them "that's an Army helicopter."
Instead of the unavoidable chance of getting shot down there is the continuous risk of mid-air collision.
Of the two ends of these conflicting requirements, I don't see that the DoD will want to budge, and I don't see a safe resolution that works without ADS-B and other broadcasts.
At the least, in a national emergency, there would be no need to follow the defined helicopter corridors. The Georgetown residents can file their noise complaints if they want to.