You might want to look at the Kobo line of ereaders. I don't read Japanese but a couple of friends who do have seemed quite happy with the language support. Horizontal and vertical text along with furigana (ruby) are supported using epub3.
They do require registration but that is in the form of an email address which does not seem to be checked (
someone@erewhon.net is acceptable hence my belief that the only checking is "does it look like an email address?"). There are also database hacks but the fake email address works for me.
The newer Kobos do not have external SD card slots but the Clara HD has an internal 8GB uSD card which can be replaced with a larger card. The battery can be replaced fairly easily. There are threads in the Kobo forums about both replacing the internal uSD card and the battery.