I found your manual [Link: panasonic.ca]
to see if it had RS-232 control (It doesn't appear to) but I did see something called Auto Power On on p.29 that can be selected in the setup menu, that I assume when it senses the presence of a live video input it switches the Plasma On.
You might experiment with that and if you have a DVD that has a discrete On command that may work. I suspect that all video inputs would have to be Off and if your STB (set top box) doesn't have a discrete off then you might need a Video switcher to interrupt the video in order to make this work. It would be interesting if it auto-switches to the live video input as well