I could be wrong, but I think maybe slate may split with the heat.....worth a try thought!
Smoggy...
There is a restaurant in west Wales that serves it's food on heated slate 'plates' so the risk of exploding in fires should be minimal if they are not placed in soaking wet or freezing cold.
Using one for cooking should be the same as using a gridle or bake stone - I've cooked steak and eggs on them no problem and burgers if they are thin should cook through before the slate cools