You can also manipulate the database of the Kobo in order to fake the registration. You should be able to find the exact SQL query you need in the Kobo forums.
If you really don't want to register, not with a throwaway account and not even by faking the database, then Kobo isn't an option for you I guess...
Kindles can read books without being registered (it prompts you to but you can skip it).
But in order to build collections automatically from the tags (using the calibre plugin "Kindle Collections"), you need to jailbreak. You don't need a jailbreak to have calibre manage your collections on the Kobo.
Currently we don't have a closed-Kindle jailbreak for the latest models, but if you are comfortable with accessing the serial port you can still do it.
Whether it is easier to update the Kobo database by hand or to insert a probe in a Kindle, I leave it to you to decide.
