Revisiting this...I read your post where it's a $2M home...So, exposed wires aren't going to fly. 

1. Go to the house and measure the size of the "square" back of the cabinet.  Go to Home Depot and get a 3/4" piece of birch plywood. Cut it to the dimensions.  Sand it and throw a couple of light coats of satin poly on it.

2. Attach 2/4's vertically on the back of the cabinet, in effect  building studs.  Now you've created a void between where the existing wires come through the floor.

3. Run wires up the studs, attach gang boxes, etc.

4. Cut holes in your birch plywood for gang boxes.

5. Screw the birch plywood to the studs.

6. Mount TV.

Now you've effectively hidden the wires and have solid studs + Plywood to mount  to.