I spent some time getting to know Kyler Laird from Indiana (along with BrianTee) who's been making his tractors autonomous for several years now by hacking electronics on them, both analog and digital. He says that Agco is one of the most friendly and interested companies when it comes to modifying their stuff. He's spoken with their engineers and they are kind of excited about what he's done and don't mind at all that he's learning how things work. The only thing that gives them pause, and really most other companies pause, is the worry about liability.
John Deere of course is the absolute worst when it comes to paranoid and controlling. My cousins did a lot of work for Deere about 15 years ago, and they had to sign all kinds of NDAs and treat all the can bus codes as top secret. After that project they vowed never to do anything with Deere again. John Deere thinks they are the Apple of tractors, except they aren't nearly as cool.