In the event we use Appian as a Content Management System, how would an external

There are a few options.
1. Using (S)FTP plugin where you can configure a process to push the file to a particular location and expose this process as a web service. This will allow you to invoke the process from an external system
2. Create a Servlet plug-in that returns the document (via WS) based on your required format (i.e. base64 string or document attachment etc)

But I'd advice caution to make sure the code for the plug-in enforces security that's set at the Appian level to ensure unauthenticated users don't end up accessing the document.