Can you safely use a cast-iron grill pan as a heat diffuser

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.

My name is Tom Wirt, with Clay Coyote Pottery. I'll try to shed some light on the intricacies of clay cooking pots, especially tagines.

You can use any flameware tagine directly on the glass stovetop. This includes, Emile Henry, Le Crueset, and Clay Coyote flameware. These are pots with either a metal base (Le Crueset) , or a type of ceramic called flameware (Emile Henry, Clay Coyote which is formulated and made to take direct heat.

Normal stoneware clay pots and earthenware pots will not do this. Stoneware should never go on a direct heating source, gas, electric or glasstop. It will crack with or without a diffuser.

Earthenware ceramic pots, typically identified by a reddish clay color and some absorbency by the bare clay (typically the bottom), do need a diffuser and should be started over a low heat. They can crack if used over sudden or too high a heat. Remember that these pots were originally used over charcoal fires.

Metal, obviously is fine.

The flameware ceramic pots, are designed for direct heat and are actually especially good on glasstops as the top spreads the heat better than electric or gas. Clay is a insulator, not a conductor. Thus the heat doesn't spread much, but, with a highly liquid food like a tagine, the liquid spreads the heat. Basically a tagine is cooked at a simmer, even though the pot would take the heat. Induction stoves require a metal plate with ceramic cookware to turn the electromagnetic force into heat.

You can find more info on my blog.