You can't program the OFF button. Why can't you just let the harmony turn the TV on and off as needed? If it does get out of sync, you just hit the help button and it will ask you if the TV is on - you say No, it sends the power toggle to turn it on, you say "yes, that fixed the problem" and everything is back in sync.

If it's just turning on the TV that's a problem, you can always tell it you have a separate on/off button, then give it a bogus ON command but give it the correct power toggle for the off command. Then you can manually turn the tv on each time and OFF will turn it off for you. But I don't know why you'd want to do that.