Actually…it would be super valuable to really support custom shadow meshes not only for artistic purposes!
Its a common practice in games to have lower rez shadow meshes to reduce the cost of dynamic shadows. So you have a high quality render mesh that you see when playing the game, but the shadows it casts on itself and the world are coming from an optimized version of the same mesh.
Usually engines access this in a similar way as you can use custom LODs, but I am not aware of any support for that in Unreal. Would be highly beneficial though!
Cheers!