Hmm, good one. I don;t know that any event is generated when an update completes successfully. I don't even see anything in EMS for successful updates.
Another way to think of it though, is if you have your relationships and schedules all set up, and OCUM is alerting on lag threshold, that kind of solves the issue. You will always know that your relationships are updating, unless you get an alert that lag time is out of whack.
not what you had asked, I know. but maybe good enough to sell your boss on!