Somewhat crucial detail is that this rule is for "Force Majeure" only, meaning that skipping the first leg because one couldn't be bothered to start their trip in TUN or other cheap starting point is - technically - not covered. Not sure how they would be able to check (and therefore enforce) that, but it is something that would limit using this mechanism to book flights out of cheap origins. So if you arrive at AMS saying that you couldn't be bothered to take the KL-coded train segment from BRU (Which is a common occurrence apparently) would make you ineligible for this option.

Another thing that I am wondering about. If you are due to travel BRU-AMS-NYC but decide to skip the BRU-AMS train segment (And manage to convince them this was force majeure)... what would be charged then? 125 euro for the skipped BRU-AMS segment, or 500 euro since this is an intercontinental itinerary?

There may be situations where this could work and be beneficial. Some origins (Like TUN or CAI) are very cheap for business class fares. Even with a 300 euro fine that may still be substantially cheaper than booking with a European starting point. However.. it is not without risk. And therefore, unless you can easily bare missing the flight, it may not be the safest thing to do. Having to buy a new or alternative ticket is a realistic scenario, and something I wouldn't wish to do with today's loads and prices. It'd be stressful for sure.