Simulating a thermopile on the bench -- is this a reasonable/reasonably accurate way to do it?
To obtain a definitive answer, I recommend that you obtain the 100 gas valves and or their data sheets, find and average their optimum operating characteristics(requirements), and determine their standard deviation.
Design your simulator to provide the average optimum parameters and allow an adjustment equal to 3x the standard deviation.  This guaranties that you will be able to adjust your simulator to meet the particular requirements of any given gas valve.