I think this is exactly why it hasn't been too much of an issue so far. I certainly can't imagine anyone in this fine group lobbing a legal law bomb our way as planners. In exploring the matter a couple cases that have come up before include family members/distant relatives suing events or legal action being taken by bystanders or non-event participants. Even though there are no jeeps in the group, there may still be some on the trail that are impacted by our shenanigans. In any case, this is not the kind of stuff we want folks to worry about on vacation.
If someone rolled their rig off a shelf road and died as a result, it would not surprise me if their estate filed suit. The likelihood of that happening is low, but the impact upon the defendants would be large.
I don’t know the cost of such liability insurance. I carry an umbrella policy that I believe would cover me in such an occurrence, provided that I was an unpaid volunteer.