I would love to add it to my 1Ds3. I recall reading that once upon a time Canon wrote ML devs a strongly worded letter telling them not to touch a 1D, but a camera that old is long obsolete.

(I literally only want a raw histogram)

(I also have a 1Dx2 but that's probably a harder port)

I have been toying with the idea of picking up an old 1D. I can't remember the guy's name that I saw do this, but he had his 1D modified to use a PL mount instead of an EF mount. Something about the 1D body (being thicker I guess) allowed for the flange distances to work out. He then mounted a $35,000 17mm wide angle to it. That lens was huge and could just suck in photons. With that lens, he could expose the night sky in 1/3 second exposures what would take multiple seconds on my gear. He mounted the camera to the front of his boat floating down river using night vision goggles to see where he was going. The images were fantastic. I always wanted to do something crazy like that

Canon have never had any contact with ML project for any reason, to the best of my knowledge. The decision to stay away from 1D series was made by ML team, I would say out of an abundance of caution to try not to annoy them.

The 1Ds3 still renders wonderful images but the UI feels so limited now. ML would transform it.