Kinda a sidecar for spudcon's Heck's Grandpas:
Next, riding in the front!
I don't see how these ones are steered without handlebars--I guess just by pushing on that bar on the back of the chair (like steering a lawn mower):
Doesn't count as a wheelchair bicycle, but this bicycle taxi does seem to confirm the steering speculation re the previous pics:
Replace the bicycle taxi's passenger seat with one of these, though, and you do get a wheelchair bicycle, after all (note "cowcatcher" in front to latch into taxi's DOT-mandated safety bar):
