Power and HDMI are no issue. I'm worried about messing with the mortar. Any advice? Will this be an issue, more trouble than it's worth, etc?
I have some alternatives. Leave my TV on the mantle or pole mount it to a beam in the attic.
If you drill the holes for the mounting bracket into the brickwork you shouldn't have a problem. In general you shouldn't be drilling into the mortar for fixing points. It's usually OK for one, as long as all the others are well anchored into brick.
However, if you don't think the mortar is strong enough to hold the bricks in place then you have a more serious problem - the fireplace isn't structurally sound and should be repaired first.
The main thing to worry about when mounting a TV on a fireplace that you might use in the future is heat damage to the TV. What I did before mounting my TV is get a good strong fire brewing in there for an hour or so, then get a good thermostat reading from the fireplace where the TV would be mounted. Make sure you then check that temperature against the allowed temperature range for TV operation and storage before you start drilling holes in the masonry.
Just because your TV operates on the mantle now does not mean it will be OK after it has a solid connection to the bricks, without that nice comfy air buffer in between.