[novou]

I think I recall somebody mentioning that they edited the database on the Kobo to remove the ads, though I'm not sure if brought it ad-free book-cover sleep screens, it may just blank out the ads.

Also, if someone made a custom firmware update file which packs in the ad-free Kobo's entire internal system, then you could probably turn your device into a vanilla one by upgrading with that. (until you do a factory reset) It's a safe assumption that both versions use identical hardware, but with different software running on top.

FW update files typically only include the changes from the original factory install, so that's why installing an ad-free FW update won't alter or remove the ad-system files not included in the FW file.
(This risk of inconsistency is also why doing manual FW rollbacks, or a cross-grade between ad-supported/ad-free is a pretty lousy idea.)

That said, removing the ads that way would not be the most ethical approach, and Kobo should offer a way to pay to remove the ads. I'm sure it's only a matter of (an unknown length of) time before that option is available.