If I understand the question and based upon that understanding, if you assume that the pilot was also diving then the time is 24 hours per the FAA that is only for a pilot and crew. The 18 hour "rule" is based upon sound medical advice and applies to non FAA rated persons aboard the aircraft. Exceeding 8000 feet unpressurized is the same as losing pressure that exceeds 8000 feet the pilot would be required to descend and maintain altitude below 8000 feet. Short periods of time above that threshold is the same as bouncing to a deeper point than you are not equipped or "allowed" to go. You may get away with it but never a good idea. As an FAA certified pilot I say wait the extra 6 hours and you are then outside the envelope and exceeding the 8K altitude is fine. However your pilot is required to be on O2 at 12K feet even if you are not.... This is the point were a person suffers from Hypoxia(narced in reverse) 12K feet is the magic number everyone in the unpressurized cabin should be on O2 before that. The pilot and crew are required.