How much could I safely heat a content-less cast-iron pan?
If it is completely empty, you probably can't reach dangerous temperatures with your stove. The iron itself melts at 1500°C. The seasoning can be burnt off at much lower temperatures, a self-cleaning cycle in the oven is enough for it (500°C). I have taken an iron pan to above 400°C without damaging the seasoning. (I don't know exactly how high it was, my thermometer doesn't go above 400°C). This was on an induction stove set at maximum, I doubt that any other kind of stove can produce such temperatures. 

Edit I just heard an example showing that you can take a pan on a resistive stove to above 500°C with prolonged heating, so you should be measuring it if you plan to go that high. (and if you have a thermometer in this range, you can post your findings to further limit the range of 400° to 500°C, because I'm curious). 

But if you say "no contents", be careful that it is actually just the pan. You cannot oil it, because the self-ignition temperature (of flash point) of vegetable oil is low, Wikipedia gives it at 327°C. It could just smoke terribly and throw a spark or two when you reach it, but it can also flame up and then you are holding a small brand in your hand.