There is no discrete code for the Amp / Through function. Trust me on that!
The only way to get there is through Audio Parameters each time you want to change.
What's the point? The receiver has to be on when using HDMI.
If I understand correctly you want to watch / listen to a source with just the plasma speakers. I would connect that source (cable box or Sat box) to the panel with component and L/R analog audio and to the receiver with HDMI and macro the different inputs on the panel.