PSI simply only handles one font, as it uses Zunes (MUIs) OpenPubScreen() function, which takes a struct MUI_PubScreenDesc as a parameter.
And this only has one entry for a font name:
struct MUI_PubScreenDesc
{
LONG Version;
char Name[PSD_MAXLEN_NAME];
char Title[PSD_MAXLEN_TITLE];
char Font[PSD_MAXLEN_FONT];
char Background[PSD_MAXLEN_BACKGROUND];
...
}
So changing that seems to be not trivial, at least on a first look. I think, you can't use MUIs OpenPubScreen() to set the other fonts..?
I guess you should use the Zune Preferences for the specific program to modify those fonts.