I don't own a tractor, but if I did, I'd want the option to be able to 'root' the tractor and install different software on it. You shouldn't have to ask permission, violate EULAs, or violate the law to innovate.
What you would do it connect to the CAN bus and read all the data there - same as what most people do for cars - except there is a lot more interesting data for you to work with. There is not need to root the tractors, or install different software, just install your own computer, connect it to the CAN bus and do what you want. Our 10 year old computers that do this on a 200mhz CPU sell for $5000 - I'm convinced you would want something more modern that is also cheaper for your experiments.
There is one exception to the above: if you want to do computer steering you need an encryption key that we don't generally make available to the public. There is too many safety concerns to let us just give that away.
They can already steer the tractor in any direction they want, for any nefarious purpose. You're just removing their ability to automate away labor (steering). This is essentially an economic, not a safety-oriented, policy.
I have ethical concerns.
Would you buy a house with a locked room in it, if the home-builder retained possession of the key? Does the de facto ownership of the contents of the locked room transfer with the title to the house, or with possession of the key? Is the owner of the house justified in removing the lock and opening the room? Is the builder justified in rigging explosives inside the room to go off if the lock is bypassed?
Ownership means that you don't need to ask permission to do what you want. No matter how many warning signs you put up to protect idiots from their own idiocy, at some point, you are going to have to finally let someone that wants to drive off a cliff go over the edge. You have to let someone destroy their own engine. You have to let someone own their own device.