I did this, more or less, some ten years ago. It was speaker level switching, and I switched mains and surrounds to Zone 2. But here's how I'd do it today:

You say you have a pre/pro, so I assume you have left and right front, and left and right surround, preamp outputs, among other things.

The Niles AVS-2 ([Link: nilesaudio.com]) lets you switch between line level signals using your trigger.

I'd get two line level switches and set up the audio so that mains and surrounds are either on the main zone or are on Zone 2. You just have to Y out the Zone 2 audio into the switch. Remember you can easily and simply Y a signal to feed two inputs; the thing that's difficult is Y-ing together two signals.