I don’t need the ZaurusDrive feature of the newer Zaurii for the internal harddrive. So, having ext3 on this drive instead of the regular FAT32 opens up some possibilities (read: Debian install :) ).
So, here is how to turn the biggest partition of the internal harddrive on a C3200 from vfat into ext3fs:

Edit /sbin/hddmount and /root/etc/rc.d/rc.rofilesys and change all occurences of

mount -t vfat -o noatime,quiet,umask=000,iocharset=utf8 /dev/${IDE1}3 /hdd3

into

mount -t $LINUXFMT -o noatime /dev/${IDE1}3 /hdd3

These instructions refer to Cacko ROM 1.23. They might be different for the original Sharp ROM.

Now, before reformatting /dev/hda3 with

mkext2fs -j -b 2048 /dev/hda3

Make sure to backup all content on this partition before doing so.