There aren't any URLs in the user table. There are a lot of URLs in the config file, but none of them look like the one that have been recorded in the firmware download thread.
Looking at libnickel, it looks like the URL is:
Code:
https://api.kobobooks.com/1.0/UpgradeCheck/%1/%2/%3/%4/%5
The parameters are probably the current version, model and anything else Kobo think is useful. The result of the fetch probably includes the download URL.
There are a pile of methods that look like they are involved in the checking for the upgrade and downloading it. It should be possible to patch one of those to always return that there isn't an upgrade.