App-agnostic authentication for Evolution Data Server extension

I was thinking of creating an extension (Apps/Evolution/Extensions - GNOME Wiki!) for Evolution Data Server (EDS). The extension would provide data for all GNOME apps that rely on EDS.

My problem has to do with authentication: the extension would fetch data from an online provider and since there are no plans for GNOME Online Accounts to allow third party applications to add additional providers (Projects/GnomeOnlineAccounts/Goals - GNOME Wiki!) , there’s seemingly no straighforward app-agnostic way to perform user authentication. Well, there might be one - creating a GNOME extension, but I was hoping I could avoid that…

Any ideas on what I could use instead?