I think I would just opt to an "on farm lease", lease the mare for X amount of dollars and leave all the risk up to the leaser. She would cover board for the mare, all breeding expenses, foaling expenses and run all risks. Or you could offer for her to do an ET if she wants to take the mare in foal home with her and foal on her own place.
I'm doing a very reasonable on-farm lease for a 2011 foal. I am putting a pre-set deposit down to use mare and then paying board for the mare once she is confirmed in foal (it's an on-farm lease, which I myself prefer so she can stay in her familiar surroundings:-)). I am covering all the insemination/breeding fees, paying for insurance on the mare, foaling out fee, all the inspection costs for foal and vet/farrier once foal is born. There is no LFG but other stipulations in the lease that allow me to use the deposit I put down for the mare in the next season to try again. And of course I'm responsible to pay for insurance on the foal once it is born. Awesome way to get the bloodlines I desire without having to buy a mare of that caliber.
I'd recommend just selling an embryo. That way you don't tie up your mare and you have no risk of getting stuck with a foal you don't want should the lessor back out after the mare is bred. You will also be able to breed the mare yourself either later this year or early next year.