Hi Tom,
I know Myoko Snowsports have adaptive trained instructor(s). They operate at 3 fields all in close proximity, and all 3 would be suitable.
https://myokosnowsports.com/
There is a kiwi based adaptive guide in Wanaka (NZ) who works there every year. I can’t remember his name. I am an able bodied skier & have skiied several seasons in Japan with my son. Since then he has had a C5 spinal cord injury & now skis a bi-ski with tether here in NZ. I am going to Japan this coming season to scout suitable accommodation (and to ski of course!).
I know that the Lotte Arai resort has suitable accommodation, but it is quite expensive. Possibly the best field for adaptive skiing is Myoko Suginohara as many of the runs are wide open.
Another region is Yuzawa. The Japanese para ski team train there but I don’t know of any english speaking instructors or suitable accommodation.
Hiring equipment could be a problem unless Myoko Snowsports have some. I’d recommend contacting them & they might be able to advise you.
Good luck, be interested to hear how you get on
Regards, Guy