Downhillers! Tandems! Motorcycles!
Actually, for cycletruck use, I think veganbikes was on the right track: use what well-known, name-brand cycletrucks use.
The LarryvsHarry cycletruck (rated at ~400lbs) uses the TRP Slate EVO brakes.
The Surly Skid Loader cycletruck (rated at 400lbs) uses the Tektro 4-piston M745.
The Riese&Mueller Load 60 cycletruck (rated at ~480lbs) uses the Tektro TRP C 2.3 cargo brake.
The Trek Fetch+ 4 cycletruck (rated at 550lbs) uses the Tektro HD-T737 4-piston.