I could see a use for it myself - I'm a stroke "survivor" and my right leg is mobile but really not very usable. A 'device' like this could strengthen muscles in my right leg, while not being another bullshit static bike that has no real benefit. I could, in theory, actually go somewhere. And not take 3 days per mile!
For steering, however... (my right arm and hand is deadweight)
Could perhaps put cable/hydraulic bike brakes on both rear wheels, in a position where they can be operated by your left hand individually or together? It seems like this should be possible.
This assumes that the pedals are connected to both wheels via a car-style differential so that the rear wheels aren't locked together and steering is actually possible.
A quick google for 'tricycle differential' returns a bunch of products that are probably in the right power range and would do the trick. Still need hand braking to actually achieve steering.
I like to think of it being only connected to one wheel with the other one free, so you can spin around in circles until you do a barrel roll. It's the only form of euthanasia approved by insurance companies in the US, trademarked funthanasia.