Can a treif chicken grilled directly on a kosher frying pan with no oil or anything be kashered?
It may be kashered with ליבון חמור.
There is a debate in the poskim as to what level of heat is required for ליבון חמור. According to some, it is sufficient that the pan be heated to the same temperature that it was when it absorbed the issur – based on the principle that “as it absorbed (the forbidden food), so it spits it out.” According to others, however, it always needs to get to around 650ºF-700ºF, because the operative principle here is that the heat, rather than extracting the absorbed food, burns and destroys it where it is, and to do so a very high level of heat is required. On this debate, see Pri Megadim (Eshel Avraham) OC 451:30 and Iggrot Moshe (YD 1:60) on the machmir side, and Minchat Yitzhak 3:66 on the lenient side.
Using an oven is not an option in this case, according to either opinion. An oven can maximally get to around 500º, not to 650º-700º. This means that it would not count as ליבון חמור according to the stricter opinion, which always demands 650º-700º for libbun. It would also not count as ליבון חמור for those who are more lenient and who permit libbun when the pan is heated to the same degree as it was when it absorbed the food. In our case, the food was originally absorbed into the pan when it was being used on the stovetop. Thus, the pan cannot be kashered in the oven, since it will not be able to get as hot as it was originally on the stovetop.
In short, then, a pan used on the stovetop with non-kosher food would have to be koshered on the stovetop. I think leaving it on for 30 minutes so it gets fully hot is the right approach.
In cases where a pan was used in an oven, and where the pan would be damaged if one tried to kasher it on the stovetop and bring it to 650-700, one can rely on the more lenient opinion and kasher it inside the oven. In such cases, I would wait 24 hours. Generally this is not required for libun, but given that we are relying here on a more lenient opinion, waiting 24 hours will lower the level of issur. In addition, it is advised to wash it with hot water and soap beforehand, which can also provide an additional basis for leniency.
As to the מציאות, see here.
Note: Chametz is different. It’s היתרא בלע and only requires ליבון קל. Although some are machmir.