jayclay and mrhappi, I suggest that maybe your bakes need adjusting if you have manual over ride brakes as when correctly adjusted they should lock even on even ground and stay locked while reversing.
Fishing, I have a rather large cable tie that I put around the reversing lever/catch, a little loose. When I need it I flip over the catch bring the cable tie over and it stays in place, remove it and the cable tie just sits there. If I think about it tomorrow I will take a couple of pics for you.