Like the previous poster said, I would try with the hotel. We live in Japan and I have never seen an ECV here. Anywhere. At the TDR, I have see a few wheelchairs, but never an ECV.
Now, I don't know what your mobility issues are, but the TDR parks really aren't that large. Nothing like walking at WDW. Are you visiting other areas of Japan? Our saying since moving here has been "Now we live in Japan. And in Japan
you walk". If you need the aid of an ECV for everyday, Japan might prove to be a tricky place for you to tour.
Good luck.