If the render engine has a 'thickness modifier', you could use that of course. Correct me when I am wrong, but AFIK neither Firefly nor Superfly have such facility. In Firefly you could use a displacement map 50% grey, and overlay a black polyline over the edges to be hemmed. With Superfly you have no such options.

I am working now on a small script called 1-2-HemIt that could do the trick.