I would he more concerned with power line borne noise than the raw magnetic field. Unless the magnetic field is strong enough to attract stray tools and fasteners, don't worry about picture distortion on LCD's. You can experiment with this. I have a flashlight that can support its own weight and more. Holding this up to my LCD doesn't effect the image. You can explore AC fields by wrapping a current carrying extension cord around the monitor. Use multiple turns because your extension cord is not likely to carry 400 Amps. This wrap will couple more energy than a simple parallel conductor a few inches behind the TV.If you are worried, wrap the conduit with a piece of grounded copper flashing. This is much easier to find on a construction site than a large sheet of MuMetal. ( I have no idea what the inspector might think about this.)I agree that the big risk is from aggressive lag bolts.