Sit upright. It amounts to air brakes. Something similar could possibly be rigged on the cart.

The problem with dragging the brakes downhill is rims get hot and brake pads get hot. Brake pads melt. Much is going to depend what kind of hills are attempted. Short hills and the dominant effect will be bicycle brakes are very well air-cooled. Long hills could be difficult. The suggestion above to use air brakes will work fine in a wooded area, in wind-swept terrain it could be a handful.

What is being proposed is quite a challenge. Heavy duty transport by bike has historically only been well developed in coastal plains.