Do you have a large mantlepiece above the fire that juts out quite far? If so then that will deflect heat from rising up towards the plasma.
If you are wanting to bolt the plasma to the chimney breast then it shouldn't be too much of an issue regarding heat through the bricks.
It would take quite a bit of heat to make the chimney breats bricks get hot enough on the outer side to cause any problems - the main issue is to make sure you chimney breast bricks are in good condition to take the bolts and weight of the plasma.
Also there is the issue of optimum viewing height - on a personal note I think above a fireplace is too high, firends of mine hung theirs above the fireplace and then took it down as it was too high - they then just stuck it on a pedestal stand.
Its up to you I suppose - other people may have other ideas.