Is it true that if there are any rules ML would be breaking by modifying the 1D X, it would be the same as when modifying the 5D3?
Of course, IANAL....but if ML is doing a clean room reverse engineer of the hardware, and are not enabling the un DRM'd use of copyright material, and if they are to a lessor note, not breaking any encrypted software owned by Canon on the hardware, they they should be perfectly within their legal rights to 'hack' the hardware and run whatever software they want on it.
You buy a car...and you can do pretty much what you wish with it, and the manufacturer can't touch you for those actions. Largely, Canon can't stop you from doing what you wish with the hardware once you buy it.
If you want to install and run completely new software on it, your perfectly legal to do so.
There is no difference I can see, between doing it on a 5D3 or a 1Dxyz.
However, as I posted before, often it doesn't matter if the company has legal standing or not, they can bring suit, and it is a contest on who has the deepest pockets for $$ that wins in the end, by dragging on the case endlessly.
But we need to remember, that at least to date....when you BUY hardware, nothing prevents you from opening it up, playing with it as you see fit or enhancing or destroying it.
It is YOURS.
Disseminating information or means for others, as long as it doesn't involve enabling copyright infringement, should be perfectly legal too.