Can I use cornstarch in my banneton?

I'd simply use AP or bread flour.

gary

You can substitute sorghum flour well for rice flours in most recipes.  Depending on the recipe, you can also substitute corn flour or a mixture of high protein gluten-free flours like millet flour cut with starches like tapioca starch or potato starch. Some recipes take well to using instant potato flakes which has been added to hot water or milk to thicken it before using. High starch and high protein flours perform differently in recipes. Rice flour is in the middle. You can also substitute any gluten-free flour blend. Most of them are mostly rice flour anyway. 

Keep in mind all gluten-free flours have their own properties that make them good for some uses and bad for others. A recipe that calls for tapioca starch does not perform well when spelt flour is used to substitute, for example.That's why most gluten-free recipes contain mixtures of different flours.

Sorghum flour is still a great substitute of choice for rice flour (in most recipes). It is higher in nutrition, neutral in taste, is inexpensive. Is similar to rice flour in how it works in recipes with good performance.

Seeds, nut flours or finely chopped nuts, dry bread crumbs, rye flour, wheat or just about anything except chocolate sprinkles works.  Dried vegetable flours. Oatmeal or grain flakes  Fruit tends to burn. Herbs, edible flower blossoms/petals, sesame seeds, linseed flour, poppy seeds, whole cabbage leaves...

baskets and keeping the dough from sticking but rice flour is the best.  Potato starch works too!