[petsfed]

In all 3 cases, the forward motion is ultimately provided by human power. So, even if you (for some reason) decided to motorize an elliptical bicycle or treadmill bicycle, the straightforward requirement is that its still mostly functional without power provided to the motor. I feel like an electric-assist handcycle should already exist, but its ultimately the same question. "Does it still work nearly as well after the battery is discharged?" I'm not super fit, but I can definitely hit 30 km/h on my road bike, so pedal assist to get me to 30 km/h is not beyond the reasonable limits of the average user.

You can certainly put a motorcycle into neutral and sort of scoot it around, but to make it usable in that configuration, you'd have to mess with the overall geometry to the point that its not comfortable to ride as a motorcycle.