@bluen: Assuming everything's in the right place (tweak the source path of the backup accordingly):
Code:
mntroot rw
cp -f /mnt/us/linkfonts/backups/f394b45e7a2716944ea2c0aeb5c008aa-arm.cache-2 /usr/var/cache/fontconfig/f394b45e7a2716944ea2c0aeb5c008aa-arm.cache-2
mntroot ro
should do the trick.
(Won't help if the backup was done *after* the file was changed/corrupted, of course, but it's worth a shot).