Is there any other way to roast tomatoes - besides in the oven, for 2 plus hours - that would use less energy? Would a solar oven work?
What is your temp for roasting them at? You can roast at a higher temperature, like 425 Fahrenheit for less than 20 minutes. An entire 2 inch metal hotel pan filled with cherry tomatoes finishes at less than 5 minutes at that temperature.
Make sure your pan is not crowded. You can also sear on cast iron and finish in the oven.
I'm not sure about the comparative carbon footprint here but you can roast them over real wood charcoal (hickory, cherry etc.) in a lidded outdoor grill. Even a Weber kettle will work. Either lay them out on a sheet pan or wrap in foil. Puncture the foil enough so that they don't stew as opposed to roast.
Not sure what your living situation or geographic location is, but you can do it outside in the sun. If you've got a good sunny day, lay them out on a picnic table on half sheets (or the equivalent). Sprinkle with a little salt to help draw out the moisture. It is critical to keep them covered with a netting or anything that will keep the bugs out! A good trick is to use cheesecloth...BUT you want to keep the cloth off the surface of the tomaters. Essentially, you need to build them a mini tent. Depending on the temperature, it can take a couple hours to all day.
Wood fired BBQ or Wood fired fireplace.
But I wouldn't cook tomatoes in a cast iron... an ovenproof dish, yes, like enameled cast iron, but not a seasoned, black cast iron as it will destroy the finish.
Roasting them on a grate rather than in a pot seems like it would work. I have had enameled cast iron get too hot and pop the enamel off about twenty years ago.Foil seems like a likely candidate but wouldn' the aluminum and acid react?
Oh, to be clear, I meant in the oven for an enameled cast iron, not directly on a grill. I roast tomatoes in foil pouches on the grill all the time with no reaction.