Yep, the Fonts hack will switch the fonts *everywhere*, including in Kindlets. And it allows you to switch the serif/sans serif/monospace & CJK/fallback families, so, yep.
(And, yeah, dumping new stuff in java/lib/fonts & tweaking the java font prefs & fontconfig to pick those up is the gist of it on the K2/3/4).
@twobob: The fc-cache stuff is specific to the Touch. The ALT_FONTS thing is still doable on those older devices, though: cf. this thread
And, yeah, the Kindle handles OTF fonts. It might horribly choke up on some fonts for no apparent reason when using the system's FreeType lib, though.
As for bundling/using a custom font in a dedicated Kindlet on a vanilla Kindle, no idea, sorry