I was over thinking the method of rotation, mounting a rear facing rear wheel to mesh the cassette with shaft and transfer rotation to wheel above. Clamp a headset bearing in a 31.8 stem perhaps and you have a floating holder for shaft. Mount a rubber cone(thinking Arthur's Kong?) on bottom end resting on outside edge of tyre a la tennis ball, fix a 700c rear wheel and hub to top and it will freewheel as a bonus..................
Metal pipe replaces broom so drilling and bolting possible in order for transport and assembly trackside.