Yours is one of those questions that nobody, or very few people at the most, has ever posed himself. If I were you, I'd get in contact with a reputable clamp manufacturer or seller and ask him. If he doesn't know, nobody will.
If you don't know any clamp manufacturer or seller, get the yellow pages of the city where you live and look under Welding Material.
Giovanni S. Crisi
Sao Paulo - Brazil

You mention spot welding, then talk about "hot-wire GTA". I know very little about spot welding, so help me if I am falling off the wagon here, but are you talking about hot-wire Gas Tungsten Arc Welding? (GTAW)

At any rate, the basics should be the same. The only thing you need to be carefull of during the calculations and measurements, is not to assume a linear relationship between voltage, current and resistance for the entire circuit, because the power sources will ensure non-linearity especially if you are working with a constant current power source. You can however assume a linear relationship in the clamp itself.

I suggest you do the following:

While the welding operation is taking place, measure the amperage. (I) Then measure the resistance (R) of the clamp arrangement without any current passing through it. (Switch off your welding machine.) (The resistance here could potentially be very small, so you may need a very sensitive ohm meter.)

Your power (P) loss through the clamps will then be:

P = IxIxR ----- (I squared times R)

Remember that resistance changes with temperature. You therefore need to decide at what temperature are you trying to measure the power loss, and measure the Resistance at this temperature.

Hope this helps

Niekie Jooste