A client has commissioned me to build this functionality and I'll almost certainly be releasing the code into the wild once I've done in a few weeks. I'm sure I'll revisit every decision I've made several times while building but my current plan is below and I'd be really interested in any feedback anyone has. I'm obviously building to a specific use case and budget so I'm not expecting to produce something that works for everyone but it would be good to create some shared value.
If you're interested I've posted my spec in the Moodle issue tracker: