I think that there is absolutely a market.
I think doing back to back 3 month leases of GP horses to help riders get 80% at first level isn’t very thoughtful to others in the area or to the horse. However, 12-24+ month lease to help riders develop their feel, accomplish predetermined goals, and gain skills for their next horse sounds amazing. Depending on the area it may be very feasible to lease a horse out to a series of riders at the same barn and the horse has a consistent environment for many years.
Currently, I am desperate to lesson on a schoolmaster to develop my skill while I am horseless. I have found a single lesson horse available in a 45 minute radius who can school at 3rd+. Having more schoolmasters out in leases or in a very specific lesson program would be incredible for riders like me who want to develop feel.