Thanks for the link to SonyFont, it's a great tool but unfortunately it didn't produce a font that would show on the reader.
I tried to convert Liberation Sans Regular to sans (tt0003m_.ttf) with it. I've tried other fonts as well, like Verdana, but still the font from the firmware was loaded instead of the one from test-startup.
I ended up extracting tt0011m_.ttf and tt0003m_.ttf from the firmware, adding the missing characters, and putting these updated fonts in test-startup.
This time it worked, I have all the Romanian characters that I need, everywhere, in TOC, LRF, book list, etc.
All I need now is time to read