My approach was to wrap the mount binary itself. If mount is called with -t vfat and the mount fails, it will fall back to -t auto instead. You can see it here
https://github.com/frostschutz/Kobo/...fat/sbin/mount
As for the mount, it's called by various scripts, and nickel itself.
I'm not actually using this hack myself. It might do more harm than good - nickel used lazy umounts (which don't umount at all, if the filesystem is still in use)... ext4 might not take it well.