Hi Jim!, Brian!
Just a thought, plywood laid flat across a few metal studs, could pull loose in the middle and the TV could droop a bit. Toggle bolts in the 1/4" variety would be my choice of weapon in this situation. With or without the plywood, toggle bolts through the side of a metal stud would hold a good size TV.
Hey, it works in hospitals across the country. Through bolts from one TV to another works in hospitals also.
As InspectorD says though, wood inside the wall would work best.