The GGP suggested "officially transfer the single extension to a new owner" which you can obviously already do (by giving the new owner your account, if nothing

The GGP suggested "officially transfer the single extension to a new owner" which you can obviously already do (by giving the new owner your account, if nothing else), and "tell the user the ownership changed and offering them a chance to uninstall" can already be done by any extension that has any sort of UI. You don't need to "[transfer] an extension's user base from one extension to another".