The quickest and dirtiest solution is to:
1. As root, edit the /usr/sbin/saveconfig script while booted to the livecd
2. Make the following changes (additions are underlined and bold)

# Directory selector
for i in `awk '/^\/dev\/[hsu][db][a-z].*\/mnt\/[hsu][db][a-z]/...

3. Save the file and go through your save config process like normal (your usb device should now be available to select)

4. When you reboot, you will have to explicitly state the location of the backed up config
ie instead of using the cheatcode myconfig=scan, use myconfig=/dev/uba1 (or wherever your config is stored)

That'll work