FWIW, as a basic answer to my own question (in the hope that this might help others), we have used the Webhooks plugin that give us some basic functionality. Now, the very act of a user viewing a page "hooks" into our middleware web services that allow us to do something. We can use the parameters passed by Moodle (specifically things like page reference ID and user ID) to enable us to perform a specific action.
I'm sure there are better ways to do this but this is a start.
Figuring out plugins will likely be on the cards at some point,