I would guess that people might have tried this but I thought that I would post it.
Been looking for ways to improve workflow for compositing.
Using Inkscape you can create a path from a bitmap. Using bitmap trace. So put you green screen character in Inkscape and trace it and then import it in Blender.
You could then cut it up and use it for rotoing. I would guess that it might be as easy to just use the new awesome matte tools in the nodes.