Now that you mention it, a beach dolly using the new Polaris Airless Tires is build able and might work. They are probably designed for a heavier vehicle so they will be too stiff. You might be able to cut some of the supports out. I have seen some smaller ones (~12 inch?) overall diameter advertised for go karts that would handle about the right weight but I have not seen them actually for sale. I have been looking for a set to build a 4 wheeled Marstrom style dolly.

As for filling the tires with fluid, I saw that once. In the days before Slime, a guy reasoned filling the tires with anti freeze with stop leak would extend the life of his old beach tires. They were too heavy, they didn't roll, his girl friend was sitting on the tailgate with that look on her face and we were drinking beer and laughing.