There was a feature request for this, but it seems to have been archived. Since Zwift is looking to expand into new sports (rowing has been “coming soon” pretty much since the dawn of the game), how about taking paracyclists and wheelchair users into consideration?
I’m not a user of either, but I have maximum respect for people who are. There must be a decent number of competitive handcyclists who use turbo trainers, and it would surely not be beyond the realms of possibility to work with a hardware manufacturer to customise some rollers to suit a racing wheelchair.
The vehicles would follow the same rulesets as bikes, just with slightly different parameters (handcycles would be faster downhill but slower uphill for example).