Currently user notebooks are located under the
observablehq.com domain. I have a use case which would warrant a subdomain. Specifically my notebooks are frequently viewed by users using the Brave browser which has a feature that allows users to tip sites. In order to do so the publisher of the content needs to register the site with BAT and verify through the domain they registered.
There maybe another solution here, which is observablehq.com is the top level registration and splits the tips with owners of the particular notebooks that has been tipped.