Being an insurance man myself, my first thought is the property owner needs to get the biggest umbrella policy he can get. Although i'd say 2 million should be enough, you never know. But huge risks here.
I have many ideas here, some a great, some are just kind of funny.
My initial response was, if people are coming mainly to ride on the "track" part if with the jumps, is it possible to somehow fence it off(even with cheap stakes and chicken wire, and put arrows to 1 main entrance where you charge for entry and have them sign a liability waiver, i understand you dont have the time to sit there and monitor, but an idea none the less.
I like the idea of fencing off the entrances or gating them(rather than cabling).
As much as i like the idea of nails, or spikes to pop tires, its not logical. People can sue for anything these days
I like Mosh22's idea, alot. although i hear what you are saying about the red tape and the local pd requiring a ton of paperwork, but it may be worth it.
Just sounds like a tough situation. Has there been any advancement in this?
Colin