Yes, this seems to be the case. If I can use this external server as an authorization source, I can do what I need. Trying to work out how I can send an API call using variables from the initial guest authentication at the moment. The URL requires the following:
http://<ip>/librarydatabase/<library_number>
I've got the library number from when they registered on the portal but can't work out how to use that as a variable. I tried using %{GuestUser:barcode} but that gets sent to the server like that, rather than CPPM replacing it with a variable.