I wish I could help, but I don't have enough knowledge of shaders to be of assistance. I've also been trying to achieve a similar result with pure 2d and panels, (without 3d). My original concept was to apply a shader to a single black panel that is the same size as the 2d level. Then somehow cut out a feathered circle around the player as well as around all light panels that are placed in the 2d level. But I've recently had no luck in getting it to work correctly.