I figured out how to install OptWare on /mnt/us partition (in an image file):
1. Create an ext3-image file on a Linux machine (e.g. 500MB) and copy it to your Kindle /mnt/us
dd if=/dev/zero of=./local.ext3 bs=1MiB count=500
cp local.ext3 /media/Kindle
2. Over UsbNetwork or in KiTerm:
mkdir -p /mnt/loc
mount -o loop,noatime -t ext3 /mnt/us/local.ext3 /mnt/loc
mkdir -p /mnt/loc/opt/etc /mnt/loc/opt/share /mnt/loc/opt/lib /mnt/loc/opt/bin /mnt/loc/opt/tmp
ln -s /mnt/loc/opt/etc /opt/etc
ln -s /mnt/loc/opt/share /opt/share
ln -s /mnt/loc/opt/lib /opt/lib
ln -s /mnt/loc/opt/bin /opt/bin
ln -s /mnt/loc/opt/tmp /opt/tmp
3. Now follow the aleksandyr's instruction (instead of "cd /opt" use "cd /opt/etc")
4. Instead of "ipkg" use "ipkg -t /opt/tmp"
Since the root partition remains read-only, one can be pretty sure that it will be not overwhelmed!
If you do a restart, you will have to mount /mnt/us/local.ext3 again (but you don't need write-privilege for that).
You can also change in ipkg.conf (will be in /opt/etc ) the destination to "dest root /mnt/loc". Then new directories (like man or games) will be created in /mnt/loc/opt