So I'm thinking I should repartition /dev/hdc3 into two chunks. One I will make VFAT to preserve USB HD, and the other ext3 to permit the symbolic link to /hdd3/Documents to point to a journalled file system.
I've done just that, because I wanted to put a Debian/PocketWorkStation install on the hard drive to test it. I didn't want to lose the VFAT partition either, because it can be very handy at times, so I kept the 3rd partition on the hard drive as a VFAT partition, and created a fourth one with a Linux format (ext3).
Actually, I backed up everything on /hdd3, deleted the partition, then re-created a third (VFAT) and fourth partition (ext3), then created the new mount point /hdd4 (you have to remount the root partition with the rw option instead of read-only for that, and while you do this you can also alter the rc.* files to reflect the change.
Note that you'll need a copy of the fdisk utility that works with the Zaurus (I remember reading that the version provided by default (if there's any) is broken and won't work), otherwise I know that the needed tools (mke2fs to format the new ext2 partition, and the tool to format the new VFAT partition) were provided with the Cacko ROM I'm using.
You can have a look here:
http://www.eleves.ens.fr/home/leurent/zaurus.html and the first and second paragraph will give you some help on that subject. I've just added stuff in rc.rofilesys for the /hdd4 partition instead of commenting out the VFAT stuff for /hdd3 like Gaëtan LEURENT suggested on the above-mentioned link.
I can upload my rc.rofilesys later if you are interested...