CpnCrunch,
CAR 602.79 - Overdue Aircraft Report.
It does not state that it must be within a 24 hour period, simply that it is to be done so "...immediately, by the quickest means possible."
The way I look at the 24-hour thing is that it comes from an interpretation of 602.77 - Requirement to File an Arrival Report and 602.79. 602.77 states that the PIC who terminates a flight for which a flight itinerary was used and with no SAR time specified must call whomever they filed the itinerary with within 24 hours.
Therefore, because the PIC has 24 hours to call their responsible person, its a given that then the responsible person has 24 hours to call SAR (or the time at which they feel the aircraft is overdue, whichever comes first).
A roundabout way of getting there me thinks.