I heard of it and I've seen it, though it was in a hatch back, not a tahoe. He/she will just have to fabricate a fiberglass enclosure and blend it with the trim panel.
There are only 2 problems with the liftgate.
First, If you have the LTZ model, forget about it, because the added weight will screw your automatic lift gate. If you don't have a power liftgate then you will just have to upgrade your hydraulic lift for your door.
Second, you can only get away with it if you are running subs that only need a small amount of air, anything more than .75 ft^3 and your run into to problems.