"The ADR requires prevention of starting and prevention of moving forward controlled by the one key."
That may be open to interpretation. The compulsory immobiliser really does the prevention of starting when wired through 2 points, either fuel cutoff, ignition cutoff or starter relay cutoff.
With the lockable brake scenario, do you want a real working lock or one to satisfy requirements on the day? To activate it you would need to have your foot on the brake when the key is turned off which allows the NC solenoid to stop the fluid. You might only do that once to satisfy somebody then dump it, or leave it on for compliance but not use it further.
Failsafe of electrical components on brakes and steering is a valid issue, there fore I wouldn't do it to steering, but would feel OK with it on brakes, especially if there was a manual release option for breakdowns etc as was mentioned. Maybe not the one below but similar concept.
While in Canberra in the early days I saw a P76 turn a corner at circa 45 kmh and the rack fell apart and locked up, the driver couldn't stop before he mounted the gutter and bent a few more bits. That wouldn't be nice at road speed.
