The situation above would only be possible when the aircraft is fitted with extremely old first generation encoding altimeter with a first generation mode C transponder. Your talking about a 40 year old setup.
Transponders have had their own pressure cell since then and it will only squirt the pressure on 1013. What the pilots altimeter settings are makes not the slightest bit of difference.
The transponder is calibrated every 3 years for IFR machines.
The only aircraft that are even remotely likely to have an encoding altimeter transponder setup will be privately owned antiques with old none digital transponder selectors, Russian or Military.
If its a digital display it will have its own pressure cell for the altitude data.
Those are standalone transponders.
For air data computer machines the transponder can get the data from the data bus in its raw state ie 1013.3 and uses that and the EFIS system converts the same data for QNH for display to the pilots. But again what the pilots set won't change what the transponder transmits.
Most digital transponders be it S or C allow you to see what it is transmitting. Just fly manual and select it and fly level on the backup altimeter and wind the QNH up and down on the main altimeter, the value shown on the transponder won't change.
There is also a theory out there which I think does come from Australian expats is that you have to be very careful and slow changing between standard and QNH and vice versa and not to use the press for standard button (which you must use your finger not your thumb to press) because the sudden change in value might trigger a TCAS event. Complete ignorance of altimeter systems, but they are adamant that you have to do it that way and change at transition.