Adding a Master Valve Using Existing Wire Runs

So I’ve got an existing 12 zone installation with existing wire runs and I cannot easily run new wires to the valves but I want to add a master valve to the system to help solve troublesome slow water leaks at the other valves. Any suggestions on how to add a master valve without running new wire. Is there a device that can run two valves off of one wire, or a wireless controller that can be retrofitted? Would love to know if anyone else has run into this issue and how they solved it.

Seems like a popular question these days, see my answer here:

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