what happens if you supply power to the set while holding down the power button, and keep it held?
i'm thinking it might be better to permanently bridge the power button so it's always "pressed" if possible, and handle power on/off for the whole rig, pi and tv, at a central switch. if you start trying to do this programmatically with an optocoupler or something, murphy's law applies and you'll end up out of sync at some point when it fails to turn "off" or "on" and the pi has no awareness of the tv's state. and anyway, you'd be adding the pi's boot time and the tv's warm-up time together to get from switch to playable since the pi's gotta boot at least a bit before you can set GPIOs, which may get annoying.