Tim,
Maybe the RWMA would be the best folks to answer your question if you email them with your specific details. If their manual provides the information you are seeking, then you might want to consider investing in one.
Or, you could check with your local community college or AWS chapter welding publications library.
But, again, it depends on the materials, and then the weld nut sizes, and not all materials might be included in the tables.
Some of the weld nut suppliers MIGHT have tables with torque values; but, you might want to consider if the values are based upon optimum, nominal, or worst case parameter inputs.
If you are contemplating production QA test limits, you might want to also consider measuring the projection set-down - but, this approach requires the proper technology if in process monitoring is desired. As you might be aware, set-down is a measurement of the amount of collapse when the projections melt. In "theory", set-down might be used as another indicator of weld nut torque limits, but this presumes the projections are properly formed and the welding process is well controlled.
At the end of the day, do the weld nuts perform their intended functions? I have seen weld nuts so well anchored that test studs had their threads stripped off during torque testing. And, I have seen others with no weld whatsoever - due to missing projection buttons. Then too, some manufacturers use automated systems to perform in process torque testing on every unit - and you are probably wise, in cases like this, to seek the minimum torque value.