In a strict interpretation of force majeure as it is defined in French law (from which the European use is derived), the event must be external, unpredictable and irresistable.

I would certainly agree that it is the first and the third--an aircraft cannot fly without the MEL and a sufficient crew is probably first on the list. However, no amount of legal gymnastics can make illness of a critical worker unpredictable--if it is foreseeable that a crew member might be taken ill, the airline is obliged to prepare for that.

That being said, the next question is whether the airline's preparation is adequate or not. If it is adequate, then the airline might still be able to avoid liability for delay, even if the adequate preparation results in delay that exceeds the times contemplated in EC261.

It's probably worth pursuing, since the nuisance value of litigating the matter might exceed the cost of paying out those passengers who persist in pursuing the claim.