I agree, diplexers would be the easiest way to get the sat signal to the receiver. One on the outside to combine the cable and sat signal for hookup to the cable in connection and one at the other end of the cable to separate them back out. They are inexpensive and would be much easier than running a new cable.
His other problem is actually getting the sat receiver to work since he hasn't mentioned whether he has a SWM system at home that the receiver is currently using? The Carryout will not give him HD either so he will have to be satisfied with SD or, like me, be back doing another modification to get the HD going. He would be better off fully understanding the way things are currently routed in the A/V system and then making decisions on what he wants before embarking on any mods.