[gshenaut]

OK, here is what I've come up with as a way to implement alerts based on these temporal contexts.

I set up a perspective for each one, showing Due & Flagged ungrouped (which is how I want to see them). For example, the one for my “Research” context is called pResearch. In Calendar, in my “Contexts” calendar, I set up repeating events titled “Research” covering my Research time. In the event, I select “Message with Sound” as the alert, and then I add a URL like “omnifocus:///perspective/pResearch” to the event. These are all propagated into the future by Calendar.

When one of these periods begins, a notification appears saying “Research”. If I click on the notification, it opens Calendar with that event expanded. I then click on the url link within the event, and it opens an OmniFocus page with that perspective, allowing a quick reminder of what I need to be doing in that context slot.

If OmniFocus itself did notifications, then it could probably be set up so that clicking on the notification would open OmniFocus directly, which would be better. But this appears to be pretty useful as is, and it resolves my original question.