For embedding fonts in the ePub, see
How to embed fonts in ePub after Calibre. Note that it does not require the "res:///", because the fonts are located relative to the CSS file within the ePub.
It might be possible to reference the system font directory, but it is more useful to reference a font directory that is user accessible. On the Bookeen Opus (for example) both might be possible, see
Fonts directory for Calibre -> External CSS. It is using "mnt/fat/" as the location for the internal memory (accessible by the user, i.e. where the ebooks go). Every device seems to use a different file location convention, and which locations are legal after "res:" depends on how mobile ADE is configured by the vendor.