slowrev - Never done it, but it sounds like an interesting and good idea to me. I get nervous myself when I'm moving even modestly loaded wagons up and down hills with a small tractor. (Come to think of it, I get nervous when moving wagons up and down hills with a BIG tractor.) Anyway, the only rule of thumb I've ever read concerning towing farm wagons (in a JD manual, and I think I'm remembering it correctly) is that for safety, a tractor should never be hitched to a wagon weighing more than four times the weight of the tractor. When I think about it, this seems like alot of weight compared to the weight of the tractor. I don't know if I would want trust a 3,000 lb. compact tractor to try and stop both itself and a 12,000 lb. wagon when they're going downhill. I'll try an find those safety factor figures again, just for reference. In the meantime, good luck with the brake controller idea.
Bernie