It’s possible.
https://support.go.goinc.jp/hc/en-us/articles/25883320478745-What-is-Ride-Preferences

But it’s not perfect options because of wheelchair specifications and driver’s skills to set up ramp. Additionally taxies would be Japan Taxi, not Van type vehicle in most cases.
https://global.toyota/en/jpntaxi/

Hope you get the answer you’re seeking soon, but just wanted to chime and say welcome to tabifolk! Glad you found our community.

there is an inquiry form but Japanese page only. if you could use translator app.
https://rent.toyota.co.jp/inquiry/form/

I am also interested in renting a WAV in November to go to those areas! Hopefully we can find a rental place!