Unfortunately, providing what playfield pixel a sprite, missile or ball has collided with isn't something that's built into the hardware collision. You have to infer it from the position of the object that collided with the playfield pixel.
That said, I'm looking into some ways to figure out the pixel through software, but the 2600 has very limited resources, so I'm working through a few methods to find out which way is the most efficient. Eventually, a Blockly block will be added for it.