I like the idea. In the past I always referred to online service and the connection to google calendars for these types of triggers, but I can see how a direct trigger through local calendars could be nice.
it is definitely somewhat tricky to do this, both UI-wise and to get it working robustly. but I’ll put it on the feature list.
what should be absolutely possible in the meanwhile is to use the Automation Server to set up your own calendar-checking shortcuts that will send notifications in exactly the right conditions.