I totaled this in my head for what it would take - receiver w/preamp out, HDMI audio inserter.

With labor, I think this would come out to be about a $1,400 project. The TV would have to be fixed at a high volume (preamp doing the volume control), which means you would probably hear an audible hiss from the TV when it was really quiet in the room. Getting the rear speakers level matched would be a bear. And the whole system would sound like you would expect = TV speakers with some surround sound.
Still asking myself why!