Wow, sounds really nice, you got my attention! My stack to achieve the same result is a bit more complicated: I installed KOreader on my device, and it can read calibre's OPDS catalog feed. KOreader has some other niceties that I enjoy, mostly wallabag support.
How do you trick the native kodo firmware? Does it work outside your home network?
There's a plain text config file on the Kobo's memory and one of the config options is the store URL. CW generates you a new one tied to your local CW account in your instance and you replace the standard URL. You can do multi user stuff that way. You can also define a "shelf" and only books on that shelf will sync if you wish, handy if you have a large collection or a small memory Kobo.
The guide on setting up the Kobo sync feature is here: https://github.com/janeczku/calibre-web/wiki/Kobo-Integratio...
You just change the API endpoint in some file in .kobo on the device.
> Does it work outside your home network?
If you expose calibre-web publicly, sure. In my experience it takes a bit of effort to do that right, but it's doable.
Its wiki is fairly good and usually answers all the questions I have: https://github.com/janeczku/calibre-web/wiki/Kobo-Integratio...