I know they make a big O-ring that goes into the split part of the rim to be able to use a tubeless tire. And yes, someone does make a smaller 'bush' type tire. Just can't think of the name right now. Maybe Nick can respond. I think he had a set on his plane when he took me for a ride. They are by no means in the same league as the AkBushWheels but sufficient for light bush flying.
Keith