Well, finally I found a user who was brave enough to try my "default epub fonts" hack (see here:
https://www.mobileread.com/forums/showthread.php?t=66102) with PRS-600. Apparently it worked for him, so I guess I can publish it.
What it does
This hack for Sony Reader PRS-600 allows you to use your custom fonts for all EPUB files without having to modify them - that is, it should work with any file you download, including protected EPUBs (not tested).
Basically, you get all the benefits of
ePub custom Fonts on 700, 505, 500, 300, 600 without having to modify each and every EPUB file.
How it works
Adobe Digital Editions, which is used by PRS-600 to render EPUB files, doesn't allow simple substitution of fonts. It does allow user CSS scripts, but the script is located in an area inaccessible to user. This hack modifies the location of the user script, thus allowing the user to easily modify it. I had to hand-edit the renderer's executable file to modify the location, which could lead to various problems, but so far all tests seem to work fine.
Beware
Flashing the PRS-600 seems like a lot more demanding and dangerous operation than it is on PRS-505 - not only because PRS-505 is here longer and the flashing process is better tested, but also because the PRS-600 firmware is all in one piece whereas PRS-505 firmware is split into two separate parts - one for e-book viewing and one for underlying tasks (e.g. connecting to a PC). It is quite difficult to trash your PRS-505, because you usually only flash the e-book part and the other remains unchanged, letting you, if the worst happens, to just reconnect the reader to a PC and flash the official firmware. With PRS-600, you could very well end up with a Reader without any firmware, in other words, an unrecoverable brick.
More beware
The hack is not properly tested. I only found one user to try it so far. It worked for him, but it remains to be seen if he was the rule or the exception. I advise against trying this hack unless you really need foreign characters.
Download
http://www.pepak.net/files/e-books/p..._hack-0.03.zip