I've been discussing in the ml forum and with the ml devs all over, thus the lengthy post:
The magic-lantern team is very careful to never redistribute Canon-copyrighted binaries or code of any sort, and the kind of reverse engineering that they do is entirely legal.
And changing some flags in the 1dx firmware via ml probably wouldn't be illegal either, Canon doesn't make you sign a contract "no 3rd party firmware allowed" when selling the camera. But as it is, even targeting a whole department of lawyers on a single person might have some impact :-o
However, near consensus among the ml devs is to leave the 1dx alone, not just because Canon says so, but also because ...
a) ml is a community project to add new features to entry- to midrange cameras (5d3 being the absolute top of the line) and only there expand upon some firmware limitations (i.e. more than 3x bracketing), ml is not there to save money for rich people - at least that's not why I contributed source code to ml.
b) it would be stupid because Canon just relaxed a bit about ml (in comparison to plain ignorance) and maybe they'll even make life easier for the devs by giving some answers that save months of reverse engineering - but on the other hand Canon can disable the ml loader mechanism on the next cameras or via the next firmware updates, and that would be the end of it. A real possibility is that Canon tries to void all warranties if they detect ml has ever been installed which would cut 2/3 of the userbase just because of fud.
To sum it up: If someone hacks the 1dx to a 1dc, maybe even commercially ("give me half the 1dx/1dc difference and I'll unlock it") it won't be via ml - personally I hope it won't happen at all, people who actually use 4k in 2013 are most probably well off and should just pay for it.
As for Canon ripping 1dx/1dc users off: A dslr is a computer, the firmware is the software, and it absolutely common to deliver the same software with features unlockable for a price (i.e. 4k on the 1dc) even if the hardware (i.e. your computer) is the same. You cannot demand "I want Photoshop Elements for the price of Photoshop Extended" just because your computer can run it. It's a legit business model, even if people might not like it. I do like it because it makes well-off people pay more, now if Canon would just make reasonable prices for the rest of us...