Even that might be a bit overcomplicated, you could probably get quite far with a bowl shaped treadmill (I mean, anecdotally the simple wheel shaped ones do fee

Even that might be a bit overcomplicated, you could probably get quite far with a bowl shaped treadmill (I mean, anecdotally the simple wheel shaped ones do feel like running, their inertia notwithstanding). If you start pitching the treadmill the tricky part is that you'd probably need to rotate the person around their centre of mass, or somewhere close to it.