Paul_KD7HB:
I have had insurance on my plant for 18 years and have never had a visit from any insurance person
I guess, then, you have never had to make a claim - presumably because you are a careful person.
If someone does make a claim and the insurers find a home-built home-programmed Arduino in charge of safety critical equipment I suspect the insurers will repudiate the claim. And if someone was injured or killed the person in charge could be charged with criminal negligence.
Far too many people sail through life assuming that what the insurers don't know won't bother them. The fine print of most insurance policies requires the policyholder to inform the insurers of everything relevant to the risk being insured. And what the insurers consider relevant is probably much wider than what the policyholder might imagine.
The wise thing for the OP to do is write and tell the insurers what he plans to do and wait until he receives their written agreement before continuing.
...R