What would likely happen if a ceramic power resistor was powered well over its power rating (for test/learning purposes and likely buried inside a bucket of sand)? By "well over", I mean 10 to 100-fold.
Structurally these resistors seem like they would be refractory to temperature (up to 1000C even) as they are made of ceramic, resistance wire, fiberglass shell in some cases, and a crimp or weld. If the ceramic did not crack, the weld did not melt, no gasses were released, and the wire did not oxidize and crack, I would expect the answer is that it would get really hot, and that its resistance would increase to a point where it would be difficult to dissipate more energy through it at the same voltage.
A datasheet for a similar resistor to the one above shows that there is severe derating for ceramic resistors at high temperature. It seems the focus of the "power rating" when dealing with a ceramic power resistor is more about the resistance value being what you think it will be when it is dissipating energy at or below its rated value.