No, you won't want them in the fireplace, yes, they would melt and be a safety
& insurance issue. You certainly don't want to poke a hole through your flue either.
You should talk to a general contractor or fireplace expert. Typically the
flu inside the brick is where the heat is, and there is likely room for
AV wiring and AC power by an electrician. You should consult a pro,
since they would likely make the smallest mess removing and installing
bricks for routing wires and running romex for AC. Make sure you list everything
you plan to use with this tv (cable, satellite, DVD, etc....) The wires add
up quickly, and you have to figure out where all the boxes will go. If the boxes
are a long distance away, the HDMI cables can get expensive. The last thing you
want to do is run a couple of wires and AC and have all the mess, then realize
you are missing some wires to watch certain content. I have two TV's
over two fireplaces, but I don't have a brick facing. Each TV has 8 AV wires,
and I could have used more coax w/ F-type connectors.