A more open wheel with some hubcaps could do some venting. There are some OEM car wheels (Corvette and others) that do exactly that.
Scoops/channels are likely to get torn off, especially a retrieve in a field that is soft, rough or tall grass.

While I have not made it a habit to check trailer wheel temps when towing, if the surge brake actuator is lubed and not bent, there should be almost no brake usage. Thus, no heat generated.