I really wonder how big of a problem with tractors, considering there really isn't anything out there for this issue.

Looking at car world, for cars even with locked ecus, you can get things like piggyback or standalone units, transmission controllers, awd controllers, dynamic suspension controllers, and so on, because there is a huge market for it both in amateur racing and enthusiast modification, and most of these void warranty.

Seems like this would be pretty straightforward to do - in the end, all mechanical equipment follows some raw control signal that easy to intercept and modify. Legally, you can even get around most restrictions by selling programmable modules and then making the software open source.

My guess is that paying John Deer for whatever they ask is not a big deal financially to most (although probably not all) farmers.