@gregl I have used this procedure a few times:
Define for example a V_VAR1 message for the query (or use any other suitable message type depending on purpose),
Make a MQTT binding for this message i Openhab (for example bind it to a switch, a number or whatever fits the purpose)
Make a rule in Openhab that reacts on this incoming message ("When Item xxxxx received update then ......)
Fix whatever logic is needed in this rule so you can send the queried info back to the sensor. If no existing binding is suitable you can of course define a V_VAR2 message (or use other type which fits) for the return message.
In the "Incomingmessage" function use whatever logic is needed to decode the returned message.
Reason for choosing a custom type of message (like a V_VAR2) for sending the info back is of course that it is easy to sort it out from the stream entering the incoming messages function. If you have no other incoming messages or the logic for the queried info already is in place then you do not have to think about this.
Hope the explanation is understandable, if not please revert.
@DavidZH - I really hate our electrical system here, but that is coming from an electronics guy, not a electrician. I like the idea of low voltage in the switches, however its AC. I would prefer to see DC, but the voltage drops might be something that they're attempting to avoid by sending AC instead. Yes, i agree about those damn rings and we also don't have the room here in the UK like you mentioned about the NL.