I own a small garage, and keep an old Toyota Starlet, for which I paid £900, as a customer loan car.
Anyone who wants to use the car whilst we service theirs' can do so free of charge, but I get then to sign a form to say that they are accepting ALL risks, including the matter of insurance, and that the car will be returned, as supplied, with a full tank of fuel.
Some people take it upon themselves to speak to their insurance company to ensure they're covered, and sometimes a small fee is charged, but most times people just sign away and off they go.
I don't see why an airfield owner couldn't do the same.