That doesn't have so much to do with fonts as with what characters
the firmware handles. If you look at what is listed in the font window
of H3Mod, that is the font used. Just a bitmap, it has nothing to do
with fonts that you know from your PC, H3Mod just convenently converts
PC fonts into H300 bitmaps. No way of changing the character set.
That said, the support covers more or less Basic Latin, Latin-1 and Latin
Extended A, see
http://www.unicode.org/charts/ What you're looking for
is some seriously weird stuff I haven't come across in any language. Not
what a music jukebox generally needs, so I can't blame iriver for not
handling it. Short of rewriting the firmware, you can't extend the character
set. Maybe rockbox has full unicode support?