I have successfully used your published instructions to lock one engine with a Rev 4 board in forward only and it works great. 

I also need to do this to a slightly older GP-38.  The board looks similar to the Rev 4 board but it does not have the connectors on the board.  The wires from the motors are directly wired to the board nad not in connectors.  Also the resistor layout looks slightly different.  Is there a way to rework this board to lock it in forward, aside from the brute force method you mentioned earlier?