You plumb two pipes through the element that are completely separate. Then you enable or disable flow through the pipes depending on the state of the element. N

You plumb two pipes through the element that are completely separate. Then you enable or disable flow through the pipes depending on the state of the element. No mixing, no loss to the surrounding material. Though this depends on good heat conduction within the material. Presumably though, the switching speed would need to be rather high on both the electrical field and the material. But if the bulk material is cheap enough that you can make it quite massive for a low enough price, I could see it work.