How to force a real factory reset from Linux:

I was able to copy the files from the system.img file included on the Vox recovery partiton to the system partition on the Vox. This leaves the Vox's system software in an inconsistent state and it will throw up errors, but it thinks that it is running an older version of the system software. This means that the Vox will download the latest update the next time you use it. This update will replace the system partition with it's own image, so any modifications that you may have made will be removed. Finally, do Kobo's normal factory reset to remove your personal data.

I noticed that the cache partition is used during the update. The update downloads two files into /cache/recovery: 'command' and 'eng.CAN.20111219.094110.40397.signed.zip'. The latter is almost identical to 'recovery_backup_signed.zip', except that it includes a file called 'update' and includes a newer version of the system software. The 'update' file seems to control whether user data is maintained when the system is reimaged. The contents of the 'command' file is a single line:

--update_package=/cache/recovery/eng.CAN.20111219.094110.40397.signed.zip

Hopefully this helps people. I highly suggest backing up your Vox's internal SD card before attempting this stuff just in case something goes wrong.