Andy's point is that a mountain bike suspension is designed for the loading and dimensions that it usually sees. It is a very highly evolved system, and it really isn't adaptable to other systems. The geometry is totally different than a cargo bike. You probably have to go with a suspension that is more like a car, with the dampers and springs pushing up on structure that isn't there yet. There are very few people in the world that can successfully design a suspension, and they aren't here.