I'm not sure a pottery kiln would actually be an optimum choice for the intended endpoint. I suspect, unless you went with direct heat on the floor, you'd find you'd have a crouton by the time it was properly brown. Making the experiment in my wax burnout oven, however, may not be entirely prudent, although the wax is supposed to fully burn off at the 1350°F.
Warpspeed's notion of making a 3kW or so two slice toaster, however, I think has a lot of merit. Cheap ones these days often run 700W or so, while better ones exceed 1kW and are still only OK as they are. As for solar usage, as long as your inverter can take it, it might well actually use less power overall, and give you a nice, crisp outside with a soft, moist inside instead of a crouton. Get a nice, dense array of heater wires and tune them to a nice, bright red hot. You might be able to just rewire the board of an older toaster and let the bimetal strip switch drive a relay to handle the higher power