My friend has been a police officer for many years. I told him about the "gray area" that law enforcement may not recognize the ST plates as legal. His response was this: if the trailer, the state issued plates , and the state issued registration all match there are no issues. He thinks he will do it with his trailers'

I have to chuckle at the comment about the State Police will issue tickets for something that their employer approved Isn't that like the saying "you shouldn't crap where you sleep"?

I don't know how current this is:
Exemptions

Wagon trailers, and any other trailers and farm implements that you use in agriculture-related work, need not go through this safety inspection in Illinois. Also exempt from the safety inspection requirements are semitrailers, pole trailers, and trailers with a gross weight of 5,000 pounds or less, including the weight of the vehicle and its load. If you have a house trailer that you use for your living quarters, you don't need a safety inspection. Registered Illinois government vehicles also don't have to go through the safety inspection.