What can I do about radiant hot water tubing above an electrical panel?
Radiant tubing is secured to the underside of the sub-floor. It is up in the joist bay. This tubing is ABSOLUTELY legal above a panel. Most homes I do these days have hot water radiant and ALL of them have tubing in the joist bays above panels.

This inspector is a renegade and sounds like he likes his power. Also, there is NO "3 foot clearance rule", whatever he thinks that is. 

I'd politely ask him for a verification of exactly what code you violated with the radiant tubing.