Originally Posted by LapLap

Hello there,
I once spent a year in a wheelchair and rather longer than that in recovery (where I needed a rollator). Visited Japan a couple of times in that time and got a refresher taste whilst wheeling a kid's buggy around.
I don't think there are any other regular contributors to this forum who have used wheelchairs in Japan.
However, your question may be much easier than you're expecting.
If you need the bench just to be able to get into the bath and access the shower, you may find you won't need it at all in Japan as the bath rooms are generally "wet rooms".
Having said that, there is a wide variety of bathrooms (and I'm not including those specifically built for those with disabilities) and some will be more convenient for you than others.
In the cheaper hotels there are two main issues that come to mind:
1. The initial door step. The bathroom is a prefab wet box. The whole thing is like a bath tub that can be flooded and the doorway is at least 10cms above floor level.
2. The actual size of the room. Once stepping over the threshold, the amount of space available could be 45cm square. Cheaper hotels have ensuite bathrooms where the toilet/bidet, sink, shower and bath share a space under 1.5m2.

Generally, the more expensive the room, the bigger and less like a box the bathroom becomes. But the raised door ledge problem does occasionally present itself.

The Strings Hotel near Shinagawa has bathrooms with separate showers that have no steps or obstructions on the floor. Shinagawa also gives excellent access to trains. You would need to contact them about a bench though.

Benches do come as standard in most Bathrooms, but they are very small and close to the floor.
You may find that you can sit on the edge of a bath and shower that way without the need of any equipment. There's no need to prevent water from falling onto the bathroom floor. If you can handle the initial step into the bathroom your options may be much wider (and cheaper) than you might be expecting.