Are you concerned about the long-term effect of switching 20mA on the switch contacts in the probe? This is the kind of thing that I might have worried about until I had an interesting conversation with a friend who had spent many years working with industrial control systems. His comment was that using 24V was not just useful for noise rejection but the slightly higher switching currents also helped with long-term contact life and reliability. Switch too low a current for too long and the eventual oxide build-up could lead to poor contact but the higher currents (but much too low for arcing to occur) helped break through any potential oxide. Not sure if it's relevant here but I thought I'd lob it in anyway!

I spend many an hour musing on touch probes, generally based around the published designs for three-contact types, but wonder about mechanical switching reliability, where can I easily source gold wires/contacts, etc. In fact, anything to stop me actually getting on with the project...