I would stay away from the heating cable. Chameleons are made to collect heat from the side. To them light equals heat. They would not reconize a cable as a heat source. I am also unsure as to how hot these things actually get. It could cause a burn if they did get on it or to close for an extended period of time.
In the cage you want temperature differances. A warmer basking spot and the rest of the cage at various temperatures. Wrapping the cage in a heat cable would supercede this. You have to remember with the heating lamp on top of the cage you are creating a convection air flow. This means cool air is draw in, heated and then it will finally escape from the top when it gets to hot. Again I am not sure how hot these cables get but it could over heat the cage.
I think your money would be better spent on purchasing a digital thermometer/hydrometer that has the capabilities of reading two temperature zones. Any adjustments you need to make once you find out what is going on in the cage can be done pretty cheaply when it comes to temperatures. Different wattage bulbs, moving the dome reflector around...etc.
Yes you could wrap the sides I suppose. Reptariums actually have soft tray liners that are made for this purpose. Remember this will create less air flow and can heat up the cage a bit. Depending on where you live and the temperatures in your area these types of things can be very benificail at stabilizing your chameleons enviroment. If you live in a warmer area with some humidity I think it is better to build on what you are already getting for free from the enviroment.
