I am sorry for expressing my opinion . Yes of course it can be done but in this case it would not be mounted to studs it would be mounted to brick. But regardless it should still work fine. But if the fireplace will be used it is not good for the tv. And personally I would not want my TV that high.Let's stick to what the OP was asking, is it possible and concerns with doing it. I'm only saying YES and to mount it in bedrock studs.
If you want to open another thread on tv positioning go for it, it's like with you're sitting in your Lay-Z-Boy recliner (I'm sure you have one), with head resting on headrest (with now zero-head tilt, because you're looking straight at it, oh and your back is straight), and you don't have to explain the neck-stretch to tip your head forward. No ergo issues b/c you're looking dead on. Even if not in recliner, everyone says how comfortable it is they don't have to tip their head for it, try it.