I may be conservative but IMHO if the MR says OC by certain hours, in my opinion that's it - the flight that makes it go over that number is the last one without an OC. A phone call to the LAME may get you home, but I would prefer not to get into that situation. In the situation being discussed I would have the OC done before I go (or get the the LAME to extend the time requirement on the MR before leaving).
If the LAME has endorsed a + or - margin on the MR then no worries extending to that limit (but not beyond it).