I appreciate the OP referred to an external inspection by (hopefully qualified) observers, but ultimately the operating captain is responsible etc etc.
True, but especially if you operate a widebody such as a 747, it is very hard to properly see the state of the wing from inside the aircraft, especially at night. Clear ice that may have formed will be all but invisible, indeed during the pre-flight walk-around if I have any doubts at all I get an engineer to request a "cherry-picker" (high-lift truck) and physically touch the wing, the only way to really make sure what you are seeing is water and not clear ice. So a quick glance at the wing from the inside by a crewmember on a dark snowy night does not guarantee much.
In my company's procedures if you can take off prior to the shorter holdover, then off you go. If you're between the two holdover times, you can depart after a pre-flight inspection (ie one of us goes down to evaluate the wing from the inside, for whatever it's worth). If the longer holdover time is exceeded, then an (exterior) pre-flight check is required, and realistically (especially on a big a/c like a B747 or 380) that quite simply is not going to happen. In practical terms, it means back to the de-icing pad, or back to the pub!