I'm trying to set up a electrical polling field that produces 2000 V/mm inside a kiln
Without the plates in good contact with the ceramic you'll get most of the voltage across the air gap since the relative permittivity of piezo materials is typically in the hundreds. Assuming the discs will be metalized, having that in contact with plates would do. Not much remains springy at 450C.. Maybe Inconel.

Note that some references have mechanical deformation applied along with the polarization field. See, for example, US2486560 (1946).

To answer at least part of your question directly, there is little current so wire size selection will be determined by corrosion considerations. In air, things like nichrome, Inconel, perhaps pure Ni, should be considered. Copper won't last long.

That field strength is very high- you'd need in excess of 12kV which his close to the dielectric strength of air for a 6.35mm gap. Perhaps you could arrange the mechanics so that no such gaps exist, or use a different atmosphere in the furnace.

Interesting question.. mostly materials science related.