I think the safest advice -for any hotel worldwide- is to not assume that what you need will automatically be available as a matter of course. In our experience (which in Japan predominantly was at 4* and 5* hotels operated by Western chains), all hotels were able to provide a workable solution. I have no idea if this holds true for lower-priced business hotels, where rooms tend to be very small, and presumably relatively few disabled travelers stay. That said, if your needs essentially amount to a shower chair, I would expect virtually all Japanese hotels to be able to produce one.