cincinnati-kid, if "ankle-breaking holes" are truly the reason some park near you is off-limits, I highly doubt offering them insurance will change their minds. All it will do is further cement into someone's mind that this is exactly and directly the outcome of metal detecting of necessity: ankle-breaking-holes! It seems to me that that's the last notion we want to bring to the table. But that's just me.
Hey, and when you think of it: "ankle-breaking-holes" was ALREADY illegal. It didn't matter HOW they got there (by any hobby imaginable), there is already laws that forbid leaving such things, even before metal detectors were invented. So to think you can get "liability insurance" to remedy such a thing, sort of sounds like this: If the speed limit is 55 mph, and exceeding that runs the risks of car accidents, therefore, if I desire to speed anyhow, do you think I can offer the highway patrol to get liability insurance, to cover accidents-while -speeding? No, it doesn't work like that. They would simply tell you "speeding is illegal, period".