More or less what the good farmers are asking for (which is not about the code, the kernel or whatever, they are not "hackers" as much as the authorized JD technicians are not computer experts or programmers or software engineers) is just access to the "database" of parts serial number of the machine.
Loosely the way it works (simplified) is a database where the (say) pressure sensor #42 has been registered (authorized) in the operating system as having serial number #0123456789. When the sensor breaks, after it has been replaced with a new (original or verified third party) sensor, you need to update the database telling it that sensor with serial #0123456789 has bee replaced with sensor serial number #2223334445 and - of course it depends on the specific part - possibly run a "self-test" program to verify that the sensor works properly and maybe tune/regulate it.
The farmers do not want the source code, they don't want to modify it, they don't want to "hack" anything, they simply want to be able to replace a part and have the thingy work.
Going back to software, let's talk of - say - Windows 7 (yeah I know that all the rage is about Windows 10 nowadays) and its activation, imagine that instead of having one month time to activate a new install either through the internet, the automated phone call in case it doesn't work and a support phone call for particular cases where the previous two options do not work, activation was:
1) Immediately mandatory (i.e. the OS wouldn't work until activated)
2) ONLY available through a local visit of a MS agent (9 to 5 , Monday to Friday) at a cost of (say) US$ 100.00/hour + US$ 1.00/mile