There are a number of heating pads on the market for heat therapy cures and general warmth in the winter. Some are electric, but some of them need to be heated in a microwave or an oven. If you heat one of these things in a non-Kosher oven or microwave, can you later heat it in your own Kosher oven or microwave? Assume that the heating mechanism has not been specifically cleaned for this purpose (it might not be covered in splattered food, but assume it's not spick and span.
Yes, you may.
It is a completely dry and not-fatty (anyone have a better word for כחוש?) item and therefore emits nothing that can render something else not kosher. See the first Shach and Taz in YD §108, and the Rema ibid.