The only issue I've noticed with painting cast iron is that it really never "looks" like cast iron. IMO the natural look is lost with its filled with paint. Especially with the mil thickness of a 2 component paint system.
I've been thinking about trying this stuff. I read a topic posted a few months ago regarding exhaust manifold coatings. If it works with the extreme heats from the cylinder heads etc.. then it should work great on a cast iron trans. Guys seem to have great long term results.

http://streettechmag.com/2015/05/04/how-to-restore-an-exhaust-manifold/