Graphics for the loaded vehicles are pretty tricky. The biggest problem is that train sprites have 128 or 256 rotations, while vehicle sprites only have 64 rotations. If you don't render new rotations of the vehicles, then they look very weird going around corners. This already happens with the car and tank, since they use versions of the base sprites.
Adding new loaded wagon entities isn't too difficult, and you're welcome to experiment. Look in entities-compatibility.lua to see how the "big-truck" and "kr-advanced-tank" are layered on top of the flatbed wagon. To make the mod use a different entity when loading a certain vehicle, open makeGlobalMaps.lua and add to the if-else chain, for example after
elseif k and string.find(k,"dumper%-truck") and storage.loadedWagonMap["loaded-vehicle-wagon-truck"] then
storage.vehicleMap[k] = "loaded-vehicle-wagon-truck" -- Specific to dump truck mod
So that when "k" (the vehicle name) == the vehicle you want, "storage.vehicleMap[k]" is assigned to the name of the new loaded wagon entity.