I do see the relevance. The reason we can’t have a freewheel on a uni is because we have to be able to assert back pressure for maintaining forward-back balance. With the addition of the front wheel, a freewheel would be fine. Thus, use of a multi-gear internal hub with a freewheel would be superior over the Schlumpf uni hub for this contraption.
edit: I initially refereced “Schlumpf” a little inaccurately - he also does make a bicycle hub with a freewheel. My comments also do not address the braking issues - I’d probably add a rear brake with a freewheel hub.