We do offer some mantel shelves that use a hollow, boxed beam setup for this very purpose. A good example is the Pearl Mantels Shenandoah Mantel Shelf - No. 412
. However, I am assuming you are looking for a mantel with legs that would allow you to route your cabling up from the floor? Unfortunately, I am not aware of a mantel that has a hollowed out leg for this purpose.
You are correct that this would be a great product to offer, especially with the current trend of mounting televisions above fireplaces.