What about filling all rivet holes with filler, than painting with marine grade "truckbed" liner, or "anti-slip textured paint" for lack of a better term.
It would be rough on the skin, but would be non slip and you can get it in many colours.
Same crap they use on steamships, oil rigs, etc...
If you're wearing boots and gloves, your skin shouldn't really be touching the rails anyways.
I painted the inside of my tin boat with this stuff in battleship gray. Held up very well. The boat remained uncovered all year, and generally had 2-6" of water in the bottom of it from rainfall and a leaky drain plug.
I never wear shoes in the summer, and sitting in the boat with boardshorts, no shoes, or shirt, I never had the texture rough up my skin. It did get a little warm when being beaten on by the sun (assburn), but most everything does.