Editing fstab is pointless since the file is rebuilt on boot. You can figure out how to permanently mount things by following the comments at the top of /etc/fstab, it tells you which scripts build it. I can't give you a definite answer since that depends on what flavour of Maemo you have installed (stock, CSSU, testing, Thumb...). Editing those scripts is not for the faint-hearted and I would not recommend it if you are not familiar with Linux.

On the positive side, you don't have to do any of that. Just edit your /hone/user/.chroot accordingly to run ED from a partition rather than image (let the comments in the file guide you). That way you won't mount your extra partition on boot but ED will mount it on start and unmount it on close.