There are several that I linked it the Github list, including kirikiroid2 on the very top.
The ability to quickly reference a Japanese/English dictionary
Yeah, this is why I said my homework increased. But it should be 100% accessible with locally stored dictionaries for quick referencing words and idioms.
Synchronization
Huh, I'll need to actually create a protocol for this. What does koreader use to synchronize bookmarks across devices? We just use “save files.”
Good e-ink support (high-contrast menus, the ability to disable animations)
That is natively accessible, as it is a visual novel script foremost. It will also be in my priority as well, as I plan on enjoying my reading on said displays in the future.
Support for vertical text (extremely common in Japanese literature)
Since visual novels existed: yes. I also faced this particular dilemma on web browsing recently, that’ll have to relook.
anyone distributing books in that format?
Absolutely yes. There’s even a database I’m not allowed to link that contains all current VNs, in various degrees of supported formats. I’m sure you can find out the key words on a search engine and find it, without risking our instance a C&D.