Basic Auth without web form for remote services, or 'get back something like remote.php'

Basic Auth without web form for remote services, or 'get back something like remote.php': While working on an update to an ancient app of mine - OPDS Catalog - I seem to be hitting something of a brick wall which can only be circumvented in a hackish and very much unsupported/undocumented way: NC insists on using its web form whenever authentication is required without offering the option of having the client present a Basic Auth request. The only endpoint which seems to still request Basic Auth when required is remote.php as used by _DAV_: