Very good idea. However, all you really need is something like this; [Link: cypconverters.com.au] between the zone two out and an external amplifier this way you can perfectly match zone 2 with the main zone even if the main zone is in full surround mode.  You just have to remember that it is the main zone that is lagging behind zone 2 because of processing, so if you match the delay coming out of zone two you are good to go.

You can also run out of the main LF /RF pre outs and right back into a pair of analog inputs that will never be used by the main zone so as to avoid a nasty feed back loop. then select that input for zone two playback using the internal back channel amplifiers. Which of course will only work if you are running 5.1 or less, however your zone 2 line outputs will in either case be in sync with the mains at this point.

:-)