I don't recall ever seeing painted oil pans on other motors, also. It seems like a good idea (for rust control I assume) that Nissan coated the inside surfaces, until it begins to fail.
Interestingly, I just disassembled my motor, which has been on an engine stand since 1997 (all openings stuffed with something, so not hermetically sealed), and there was no rust on anything at all. I hadn't turned the motor in over a year. Whenever I did, I removed the plugs and squirted oil into each cylinder, and removed the valve cover and poured oil over the cam. Then give a few turns by hand. Since putting it on the engine stand, I've probably done that maybe 12-15 times.
When I took it apart, everything was still covered in a thin film of oil. So I am ambivalent about repainting the oil pan. It's just one of at least 1000 decision points as I try to move forward.