I had my daughter's new pony on trial for a couple of weeks not long ago.
I phoned up the NFU and added her to our policy as "on loan". I just had to give the name and address of the owner, as obviously any payment for the loss of animal would have gone to the owner.
explain to insurance company that you have horse on trial, and dig deep and pay the extra for loss of use in case it breaks big time. you really ought to have it vetted before having on trial so you know its actually sound, bloods inc to store. if it goes lame quickly you can have the bloods done to see if anything had been given to mask a problem. god i am so suspicious!! couldnt you ask the owner if her insurance will cover you for the trial period, and offer to pay the 2 to 4 weeks premium occuring for the trial period.?i would also set up some trial agreement with conditions (stabling feeding shoeing vets fees public liability and return options who collects delivers to whom etc, and also agree a price prior to trial so if you like said horse the price doesnt suddenly go up.
Yes, phone up, and say its on loan/trial for a couple of weeks. If i have things on trial i have them as my own, pay all bills, vets fee's etc. If they do, god forbid, die, then I would make sure owner was paid in full for them.