Reply To: Booking Wheelchair Seats on the Shinkansen
Unfortunately, it doesn’t seem you can book tickets with an app for JR East (North-east of Tokyo).
Instead, you can request tickets via a form now (instead of going and booking in person). However, you still must go in person to pay for and pick up the tickets in advance. (Though, I suppose if you are buying them at the same station you plan to leave from, you could arrange to pick them up the same day you leave.)
Another unfortunate point is that it seems the form is currently only in Japanese:
