I love cornbread and we make it (as well as cookies, cakes, pizza...) in our microwave/convection oven all the time.
I do turn off the turntable, but I can't imagine that make much difference. We do not use anything cast iron. For the cornbread I use a glass baking dish or one of our silicone baking dishes. For ours we've found using the conventional oven times and temps works best. We always put whatever we're baking on a rack, not the turntable.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
Wrangler Unlimited toad