ANother thought to reduce (but probably not remove) wobble in an ultimate:
you could have a solid disk wheel, with large circular cutouts (about 10
cm diameter) where the pedals normally are. In the space of these cut outs
you would have a circular ball-bearing casing with a floating pedal inside
it. Thus the pedal would always be facing down, like the seat on a
monocycle, which is always at the bottom of the wheel. You would then
be able to pedal and apply force directly on-axis and above the
support position. THe only problem is that you’d have to ride pigeon
toed and would probably do hideous damage to your knees.
nic