I am a Japanese. I hardly use Japanese dictionaries in my daily life but I did some test out of curiosity for this thread.
1. All of my Boox 60, Kindle Keyboard and PRS-650 (non-Japanese version) can display vertical Japanese sentences with furigana/ruby in pdf format nicely. I have created lots of classic Japanese books using the Aozora-Kindle converter web site mentioned in this thread. PRS-650 can not show Japanese and Chinese characters used in book titles and author names properly while other two can.
2. Boox 60 built in dictionary software with Japanese-English StarDict works much better than Stardict German-English dictionary. But the dictionary window stays open, hide one third of screen and thus makes itself useless with Japanese vertical sentences.
3. I have tested these classic Japanese books on my Androd tablet and found Mantano pdf reader with ColorDict software + StarDict Japanese dictionaries offer quite comfortable environment for reading Japanese pdf files. The ezPDF reader which is my favorite reader for German magazine failed to let me select Kanji words in vertical sentences.
4. Rooted PRS-T1 has a possibility of running Mantano pdf reader with ColorDict.
"Modify default installation target
Some apps like "Mantano Reader" could not be installed because the installation target points to: "preferExternal".
There are several choices to deal with that:"
If Mantano pdf reader on PRS-T1 runs like on my Androd tablet, you need not to buy the Japanese version of PRS-T1 for reading Japanese books with furigana/ruby using Japanese dictionaries.
I hope someone with a rooted PRS-T1 can verify this possibility.