I had my hands on probably a hundred rib cage gearboxes out of project/parts car, never saw one painted, can't speak for the smooth case boxes.
Mike, I had a few racer I prepped block for, and they wanted me not to put any color on the block, but I had to let it leave my shop looking more presnetalbe, so I would paint them with cast iron engine paint, a few of them liked it so much thats the way they left the engine that way, I have MGB race engine I'm building now that is painted cast iron, per the customer's request.