I spoke with an insurance company about a lease horse once recently and they actually recommended something along this time - I did not double check it with a lawyer or really ponder the potential implications too much, but this company said that they had many clients who required leasees to a.) hold insurance, b.) pay a percentage of the horse’s value if it is permanently/irreversibly injured outside of accident, and c.) pay a certain amount (not full cost of lease or full cost of any ongoing vet bills) per month if the horse is out of commission for an injury that isn’t an accident. Obviously there are things I outlined there that would need to be reworded into legal-ese and there are some fuzzy definitions around “accident” but apparently its been done.
And as an owner, I’d be all for it. If my horse is injured because… I don’t know, say the leasees don’t get the horse fit before expecting it to jump around because they figure -most- of the time a horse will be just fine for a few jumps even if it is unfit, and the leased horse pulls something… That is their fault, and in my opinion, they should be responsible.