I would like to use a 4 x 8 piece of plywood and bolt the brackets about a ft from each end to load a 6 x 12 Uhaul trailer. Would this work?

The brackets are designed for 1 1/2" thick material. Plywood is at the most 3/4". Unless you plan to double the plywood, I don't think it will be strong enough for a load. You could use 2x8" or 2x10" lumber and lay plywood on top. I would recommend temporary screws to secure the plywood to the lumber, just in case you step too close to the edge. Try renting ramps at the location where you plan to get the U-HAUL.