Eventually using the information provided and a bit of tweaking, I got OptWare working on the Kindle 3.
I've put the setup instructions on the wiki:
The required file is attached. Put the Kindle in USB drive mode and unzip the file onto the drive, unplug the Kindle then do (in the Kindle shell):
cp /mnt/us/libipkg.so.0.0.0 /opt/lib/
Note: The above hack is a very dirty hack, I simply removed the --passive-ftp parameter from the shared library file and added filler to bump up the filesize. This seems to work fine on testing ipkg.