The point of this feature is to provide limited access to data that webpage JS is not normally permitted to have – namely, information about the user’s interact

The point of this feature is to provide limited access to data that webpage JS is not normally permitted to have – namely, information about the user’s interaction with other sites. (Although data sharing between different origins is not always forbidden, Safari’s “Intelligent Tracking Prevention” tries to block it for origins that are used for tracking.) Therefore, making it an “application workflow” implemented by the page JS would not work, whether or not a remote server is involved.