@Isamilis: Are you claiming that USE_ALT_FONTS thing (as in adding custom font to Kindle's
Aa menu,
custom font shows up next to all the built-in fonts)
worked on fw 5.6.5?? You're not confusing it with
this the custom fonts hack which can only override fonts already listed in
Aa menu window.
Because the first one
used to work on any Kindle Touch and Kindle Paperwhite 1st Gen (up until firmware 5.3.9), without the need for jailbreak. It never worked on any firmware for PW2, PW3, Voyage, Oasis... because that hidden functionality was removed starting with firmware 5.4.0.
The second one requires jailbroken Kindle and
should work on any future firmware 5.x.x, you just have to
reinstall it each time your Kindle's firmware gets updated.
Or maybe you meant to ask about "FONT_RAMP" flag that enables user to redefine those 8 default font sizes? That is usable on PW1, PW2, PW3...