My general opinion on cattle insurance is that if you can't afford for them to die, don't spend the money on them. You'll spend a lot of money over the long-haul insuring them. Your situation is a little different with a loaner calf, but...........
The animal probalby isn't truly worth what the breeder claims it is, otherwise they would have sold it. If it was truly as valuable as they claim for breeding and they didn't want to sell it, it wouldn't be worth the risk of having it showed. So, be careful about letting them make you spend as much on insurance as you could buy a good heifer for.
Me and my family have shown heifers for people before that didn't want to sell them. And I've provided heifers for other kids to show. Every deal I've ever been a part of - both on the receiving and giving ends - is if an animal dies - for whatever reason - nobody owes anybody anything. The breeders loses his animal and you've lost your hard work and feed. The issue needs to be discussed upfront, but I'd be very reluctant to take on an animal otherwise. If they want it insured, tell them to buy it.