Rendering a prefab with material X

This must be quite simple but searching I can't find it

eg I I dont want to change a prefabs (instance's) material, I just want to render it twice in a frame, once with the normal material and then again with material X.
I know how to do this with a single mesh with 'DrawMeshNow' or 'DrawMesh' but I want to draw all the meshes of a prefab (i.e. a skinned multiple mesh)
whats the best way? In the Camera Post Render I assume but how
cheers Zed