I am not aware of any differences between wood and coal that would dictate a different design, except for the fact that coal is burned in chunks, whereas wood is burned in logs or sticks. It's easy to stack wood in an oven to allow plenty of air to support the flame; I'm not sure how I'd feed a coal fire to allow the same airflow.
I don't want to divert your thread but why do you want to burn coal? It's dirty, there are questionable health risks from high-sulfur coal, and it's probably harder to find than wood. Yes, it burns hotter, but wood burns plenty hot enough; trust me.
Have you tried fornobravo.com? They have an excellent wood oven forum.