@Xiphware and I had some back and forth today and he's overall preferring the hybrid approach. He has highlighted that he is thinking about event-triggered recording because he is interested in user-directed data capture with something like events from a client pause/record button triggering data capture.
One additional possible wrinkle I want to mention if we do go in an action/event direction is that xforms-ready is still not well-defined. We had a long conversation about it when we added odk-instance-first-load. Upon reflection, I think that the question about whether xforms-ready is fired or not on reentry into a saved form might be irrelevant in W3C XForms because the assumption is that forms are filled out while online and that there's no "save as draft" concept. Either way, we'd need to decide whether we re-introduce xforms-ready with a different meaning than what it used to have or introduce a new event.