Our house had hedges like this when we moved in. We trimmed up the front of the hedges to encourage it to grown and cover the base. It pretty much did but it took many months. They are holly bushes that also grew to 6 feet tall covering half our windows over time. This year I had had enough and took the chain saw to it. When the chain fell off, I used my regular hedge trimmer and loppers to hack off 2 feet of it. I knew the top would be naked and bare but I didn't care as long as the bushes were lowered. There is now new growth on the top! I don't know how low you need to go to get it to grow underneath and cover the bare branches. I like the idea of adding undergrowth to yours. I have a row of Katie's Ruellia below our South-facing headges. The hedges look lush along with it.
