There's an old industrial system called the "Micro lock" that was designed for forklifts & such that didn't have park brakes. Basically, it's a controllable check valve that goes goes between the master cylinder & brake. There's two types, one electric & one mechanical. Flip the switch or lever, pump the break pedal until it's hard & break is set. Flip back switch or lever & it releases. It works good when new, but does cause some problems after it gets a few years on it. It gets so it bleeds off & releases the brake or won't release at all without cracking a brake line. Not the best idea, but it's one solution 'till a better one comes along. Of course, there's always the MSHA/OSHA fix -- the wheel chock. It's a pain in the butt, but it works is cheap.
NM