Hi arboraso,
Something happened in your question that everything you wrote is kinda scrambled.
About your topology with the sensors, not sure exactly why you are trying to use SNAT, guess that maybe that's not what you are looking for.
In your case, i would give static lan IP's to each sensor, and push routes to them through the OpenVPN, so any client on this VPN would know the route to reach each sensor. You can read how to do this here.
Best regards.