They are just bitmapped fonts.
I have changed one of the fonts on my player, not the ebook ones.
Here is my software to do it:-
http://www.mympxplayer.org/rockfont2...-pm-df899.html
It is a early version yet and it only works on one of the fonts and fonts for some players are not quite lined up, but I plan to do an update so the alignment can be tweaked..