I have done many 'in-wall' and 'hidden' entertainment setups, but you are looking for a solution for a brick fireplace.
I did one where I cut/routed out the mortar in between the bricks to get to the outer edge of the fireplace, installed the cabling from the back of a wiremold box into the joints in the brick (you will need to route out more than one brick line, one for power, one for signal wires) and followed the brick line to the fireplace edge. Then I installed a corner raceway from underneath the mantel top, along the wall to the floor.
The raceway can be painted to match the wall and looks like crown molding. I used mortar patch to go over the cabling that ran along the brick line - you cannot see the difference!
Sorry I don't have a picture or diagram to show you, hopefully you can visualize what I am saying.