So far that I have heard of there aren't any allowed meshes of trapezium shapes on ROBLOX. So instead of putting two wedges and a part together trapezium meshes can be used, and games can have less bricks in them.
No, it would be just like having a head mesh. With the new feature, you would be able to place a brick, put a SpecialMesh, then from the drop down menu you would be able to select "trapezium".