I don't exclusively mod Atari but I've dealt with the 5200 inputs... I've decided to document my mods and experiments using youtube now. And for anyone curious I'm currently doing some experiments using conductive paints, starting with the membrane pads used in most digital inputs. I wasn't sure which area to share this in since it has applications beyond a specific console or pc. Eventually I'll be working with changing the formula if I feel it needs a different finish or flexibility.
I don't really have proper recording gear, bit it gets the point across -