Well, the cool thing is a flat screen over the mantle ... so here is what I did in one house....
1. Cut into wall and ran power with a properly positioned outlet using closest receptacle for the feed.
2. Making sure I had at least 6" separation from the AC ROMEX, I ran TWO HDMI cables to the niche where the electronics were to be located. NEVER run comunication cable paralle and in close proximity to AC power ... it can cause picture and sound distortion. If you have to cross the cables, do so on a perpendicular.
3. Fortunately, the wall studs were well located, so attaching the TV bracket was easy. Absent the proper location, a 2x6 horizontal nailer is the best bet.
4. As to heat .... The fireplace surround was a zero clearance model so there was no need for an add-on chase. I simply stapled the ROMEX to the studs and used cable staples for the HDMI (they do not crimp).
5. Fixing and texturing the drywall finished the job.
Everything turned out excellent! GOOD LUCK