At the moment the ORR seem pretty determined that it has to be a centrally operated locking system, although as mentioned earlier this carries its own risks in the event of short platforms or loco/power source failure.Obviously this risk can't be mitigated by placing a cover over a bolt, however, to my mind, that seems like a perfectly sensible method of work. Would it not be worth WCR simply investing their legal money into implementing this modification (assuming it meets regulatory requirements)?
However if I were in charge of an operator using heritage stock, I would certainly be putting this argument to them to try to reach a compromise. Maybe a heavier duty manual lock, rather than the current DIY store ones, with a cover and clear signage not to be touched by passengers except in emergency, and a window bar fitted.
For purely steam operated, vacuum braked charters it would be a lot more affordable and feasible doing this than trying to rig up some sort of on board power supply.