[usrusr]

The way I understand hubless wheel designs (powered or not) is that you don't build them as one big bearing, wasting huge amounts of load bearing capacity in all parts of the rotation that aren't the ground contact point. I assume that the moving part is the rim is designed as a rail, with tiny trucks (as in the rail car component) riding on it that are fixed to the non-moving part. You'd have a high density of strong trucks near the floor, some at the three and nine o'clock positions for braking and acceleration force and perhaps some flimsy guiding on top. Those trucks would not necessarily require more sealing (outside their own small bearings) than the rail/wheel contact in railroads need sealing. And moving that rail a little hubward, behind a lip that extends rimward would already get you strong centripetal forces driving out all ingress in contact with the moving part, and adding some overpressure (that you might need for cooling anyways) would help even more. I think it could all remain contactless on that first, whole-wheel level, at least if you don't design for routine wading.