If an object is set to block light, only its bounding block matters, even if it has transparent pixels. IMO lighting should take things like object’s transparency into account when blocking light.
It would be extremely costly to compute, lightning engines are always based on polygons because it’s far more efficient to do computation on vertex and edges.
But collisions masks should be usable as light blockers if I rewrite the lighting extension one day.
Not collisions masks, please. Just make separate lighting mask, since sometimes we want our object collide differently than it is lighted (e.g. rectangle collisions with more faithful to shape lighting).