Of course, just wanted to make sure the idea wasn't a non-starter.
In the "crystal ball" menu where you can add a "website ranking adjustment", there is now a widget to add the ranking to one or more lists:
It's a dropdown menu where every entry is a checkbox menu item corresponding to a list, including a "create new list" option. It works like you would expect.
There is now a list of all lists created by users:
Lists should probably also have descriptions.
Bonus feature: A user can add others as "maintainers" to their list.
Very cheap way to implement this: Lists are GitHub repositories.
When you subscribe to a list using the interface above (you are subscribed by default to your own lists), the website rankings on the list are now applied to all of your searches as if you'd set them yourself.
Bulk list editing: