I often struggle with the "as original as possible" and "this would be a nice addition" dilemma (or, opportunity?)
In this case, if you would enjoy it more, go for it. It's not like you're knocking out the fireplace and putting in a gas version that comes with a remote. We're talking about a mantle. Aesthetically, it would be a nice design feature to cut the vertical with a horizontal line.
If you want to put it in, but will struggle with your inner self about it, then don't.
We live short lives, so do what will make you happy (just don't put in that remote operated gas fireplace, OK?