I can’t see any obvious way to install new fonts (no /font directory!) so short of a firmware update, I don’t think it’s possible. It *seems* to respect the fonts within PDFs – but I haven’t tried many of them.
I think that the character encoding bug I encountered was a one off – I haven’t found any other problems within other books.
If you can find me a unicode file with Cyrillic, I’ll happily test it for you.