When you look in the Sqlite database on the Kobo which references all the books, you see the following path given for material
Code:
file:///mnt/onboard/Russinovich, Mark/Zero Day.epub
.
This makes me wonder if you could refer to fonts in the /fonts directory on the Kobo as
Code:
src: url(res:///mnt/onboard/fonts/CharisSILR.ttf)