Add-A-Wire - COMMON / C Wire to a 3 Wire Thermostat
Don't even bother with that goofy multiplexer.  Just get a small 24VAC transformer - 10 VA would suffice (common ones are 40 VA, that's fine too).  




On the secondary side, connect one of the transformer secondary wires to R. You do this on the Nest's wall plate, you don't even need to install the Nest proper for any of this.  
Now, check the phasing of the transformer.  Measure AC voltage between the remaining transformer wire and the R terminal (the place you just connected the first wire).  Note that voltage, it's gonna be around 24V, because it's coming right off the new transformer.  We just need it for a reference: it should be the highest voltage we see.  
Now, measure between that remaining wire and all the other terminals on the Nest one at a time.   All these should be equal or lower.  If any or all of these voltages are higher (you might even see 48V), then you picked the wrong wire in the first step.  Switch wires and start over.  
Now that phase is correct, connect the remaining wire to C. 
Don't connect any other wires from this transformer to anything else. Don't connect C to anything but this transformer. 


If someone rewires the furnace, changes the furnace or replaces or rewires the furnace's transformer, I would redo this phasing step in 2-3. The idea is to have these transformers work alongside each other, not "stack" to 48V, which asks a lot of the insulation on the Nest and technically violates the low-voltage wiring rules.  (I'm sure it's built for that, but why tempt fate).