I think the 2nd part (cars/people/bikes) can be achieved using the CMasking_ID,CMasking_Mask, or CMasking_WireColor REs, no? Maybe I don't fully get the idea.
As to 1st one (shadows of a/b/c) I understand it and it sounds like a valid feature request for the VFX part of Corona, but for now there is nothing more I could add. :(