<snip> Could something be rigged mechanically
Probably, but as dexter already has maybe loaded backing plates can be
fitted? I have seen smaller cargo trailers (military 1/4 and 1-1/4)
with parking brake levers, so the technology is out there.
or maybe
> electrically? Do electric brakes need continuous current to stay engaged?
No and yes. Electric brakes are actually mechanical brakes with the
lever being pulled by an electromagnetic "puck" on the inner face of the
drum. Sort of a "half disk" arrangement. Not only do they require
current to work, but constant current will burn out the puck windings.
The classic "lose/lose" situation.
Matt