I don't quite understand your question. Be more specific and you'll get help.
Fonts are referenced (by font name, not file name) not only from xml files, but from some binaries, for example LRF viewer.
So if you want to have different fonts in LRF, rename fonts (fonts, not files, filename shouldn't matter) and replace the old ones. If you simply want to have your UI having different font, then just adding new fonts to the /opt/sony/... blabla../FONT and _correctly_ referencing them from XML should work.
And remember, that it's very easy to brick PRS 600 when experimenting with firmware changes, and so far (unlike with 900/350/650/505/300) there is no simple way to unbrick it.