The firmware contains these font files, I'm not sure which characters they cover, or which font is used for what precisely.
Code:
AdobeHeitiStd-Regular.otf
AdobeMingStd-Light.otf
AdobeMyungjoStd-Medium.otf
CourierStd-Bold.otf
CourierStd-BoldOblique.otf
CourierStd-Oblique.otf
CourierStd.otf
MinionPro-Bold.otf
MinionPro-BoldIt.otf
MinionPro-It.otf
MinionPro-Regular.otf
MyriadPro-Bold.otf
MyriadPro-BoldIt.otf
MyriadPro-It.otf
MyriadPro-Regular.otf
RyoGothicPlusN-Medium.otf
SymbolStd.otf
iriverSD.ttf
iriverSMJ.ttf
The iriver*.ttf should be what the device is using for its menus, file browsers, etc. the other ones are what the epub viewer uses.
MyriadPro is what was responsible for those little page numbers on the side (which were removed in 1.29 US firmware) - I modified this font in my HideNumbers mod to get rid of them.
And there pretty much ends my knowledge about story hd fonts.
You can use custom fonts with my UserStyle mod (for EPUBs), if the CSS integrated into the EPUB is nice enough to allow such an outside style to apply. Of course you can also embed fonts into the EPUB itself, although it makes for large epub files.