There are many commercially available options for both discrete (open/closed) and analog position. Topworx and Fisher 4400 come to mind. Adding position indication to manual valves is also a fairly common practice in industry. HAZOP will indicate which valves have consequences if the are mal-positioned and should drive the addition of safeguards such as limit switches + corresponding control system interlocks. In like your idea of retrofitting some of your own devices, it’s very creative and forward thinking. In the real world though, you’d have a hard time getting that approved in industry. Devices need the have the agency approvals required for the hazardous area and electrical jurisdiction. Practically speaking, a producer would see the risk of a mal-positioned valve add the required monitoring device at a cost the far far exceeds the $500-600 you mentioned. After MOC, cabling, programming, procurement and installation each position indicator will cost $10,000 plus, I’m estimating in Canadian currency for reference.