What it does
This hack for Sony Reader PRS-505 (possibly the newer models too, but you are on your own with them) 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-505 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.
Installation procedure is described in the readme file, both for "quick and easy" and "I want to combine it with other hacks" variants. If you have any problems, I will try to help.