I think a disk brake will be a nightmare for contamination being as it's going to be in line of fire from the mud etc from the front tyre unless you have done sort of shield. Holding it on when you are riding a section that requires you to be standing up may also be problematic.
A mechanical systems with a positive lock like a pin that can be switched between engaged and not doed sound like a good solution and would be quite straightforward to make.
I dintd think you'll get the pin idea to work.
You'll either snap the pin or you'l struggle to get it to engage as you rotate it unless you heavy chamfer the hole.
Was going to suggest a back pedal lock but you wont be able to back pedal i guess.
You need a remote lock out knee?
Maybe a drum brake would be easier? More shielded, it only needs to be on or off so no real modulation needed?
Something like a coaster brake for the crank?
yes, i was thinking about the load on a pin and it would be pretty significant (but IANA Engineer) - but would something that the crank rests on directly be possible? like a bar that swings out when released and clicks into place - then when you press the catch it is pushed back into position when you down pedal?
