This was our solution.
TL;DR:  Trailer shop made a diamond-plate shelf that we bolted to the vent opening.
My son just bought his first place and I gave him one of my old home theater systems as a house-warming gift.  Room and door layout dictated that the only available location for the TV was over the fireplace, which meant that’s where the center had to go as well.  
The speaker is a Def Tech CLR3000.  It measures 8.5h x 16d x 25w, has a built-in 10” powered sub, and weighs 62lbs.  We lucked up and found a live wire under the mantel (seriously, a cut wire with a bit of duct-tape around it), which now powers the TV and the center sub.
I drew up a design, went around to a local trailer manufacturer, and for $100 they made the following.  Think it turned out pretty nice but it sure was a bear to mount.  Four lag bolts rest on the horizontal brick (and are screwed in to a piece of metal behind the brick), and there are 2 U-bolts wrapped around 2 of the vertical bricks (at the top edge).  
The weight of the shelf and speaker are primarily supported by the bolts that rest on the horizontal brick.  The shelf/speaker could be removed and you’d never know they’d been there.      
It will probably kill using the fireplace but eh, who cares.  It doesn’t get that cold where he lives anyway and personally, I’d rather have the A/V stuff.  And the system ROCKS in this room.  It sounds SOOOO good, I’m kinda’ regretting giving it away.  Oh well.  









 
 

Nice but I will never understand why you would want your tv that high up.  I can't imagine it being very comfortable to watch that thing very long.

In principle, I absolutely 100% agree and have often advocated that generally lower-is-better.  Not my house though.  Heck I was ready to close up the passageway on the right wall, move it over, and use that space but he declined.  



Ideally the center would be at the same height as the tweeter of the mains, with the screen 12* below the horizon (when viewed from a normal upright sitting position).  This setup actually works great though when reclining on the couch or stretched out in the La-Z-Boy; although it looks a little goofy aesthetically to me as well, being so high up.  Viewing distance is about 17', which floats the screen just above the iPad resting on my belly.    



This project prompted me to wonder if there would be a market for custom fireplace mantels that could also serve as speaker shelves or even enclosures.  Make them deeper at the top edge and beef up the support since over-fireplace installations seem so common now.  Na, most folks probably just use on- or in-wall speakers with those types of setups.

The TV is too high.  It should be placed on the hearth, at eye level when seated.  You would be better off swapping the TV and speaker locations.  You've already lost the use of the fireplace, you might as well get the theater right.

Pretty fuckin' sweet housewarming present.