If the Magic Lantern team can do anything with the 5D Mark III, they'll be able to remove this limit. It'll be considerably easier than with other cameras since the 5D Mark III can already split its videos up into 4GB chunks.Following the ml development for quite a while, I wouldn't be so sure if this limit is "fixable" because ml can only use the firmware hooks that Canon exposes. It is very well possible and might be even probable that Canon did more than just "disable" the continuous recording, but really prevented it.
The reason for this and why it's not eu-only might be that they want to sell their real and upcoming movie bodies at an even higher price. The good thing: At 3500 bucks, this problem won't matter to most people :-p
If magic lantern does enable this I wonder how longer videos will go with overheating?I am pretty sure overheating is not the problem - the only problems with ml and overheating were on older, obviously less power-efficient bodies (like 50D, unlike 60D), and occurred a short time after enabling specific features. A slow heat build-up until exactly 30min seems very improbable, the outside temp (shooting in the desert or in the arctic) will play a larger role here.