Metra's gallery cars - 8400 series and newer - are equipped with a 30CDW (fully pneumatic, so called "desktop" type) automatic brake valve with a companion parking brake handle included that looks and functions exactly like an independent brake valve - and they do have a J relay valve. The more brake cylinder pressure you require, the further you push it into the service zone. It builds brake cylinder pressure on all 4 axles of the coach, maxing out at 100psi. Obviously with that kind of BC pressure, it requires the engineer not operate it foolishly when it's being used outside of its intended purpose as a parking brake - but there are plenty of situations on the line of road where it is very useful, and I don't just mean throwing it on when you're coming in to a station too hot.
So far as I know, no push pull commuter operation exists in the US where an independent brake valve in a cab car operates the independent/ actuating feature on the locomotive at the other end of the train. The MU connections don't exist on any equipment and even if they did I would be concerned about the reliability of operating it - that's a long way for MR air to travel on demand rather than being constantly pressurized.