What does it say, for those of us who have misplaced our copies of RG&JS?'Radius, rod ball socket & cap, p4-22 Judging Standards Revised 2011'
If it was me I would paint it a nice semi gloss black and not worry about it nobody's gonna crawl underneath there and verify it unless you're building a high points restoration who cares!!!
Paint is not necessary to stop rusting of the cap. Oil leakage will provide lots of anti-rust protection, so leave it bare.