It's an interesting idea... I can see where it could be really cool, but I also see a case where it could backfire a bit.

If you have the rear speakers enabled, Dolby EX and DTS ES-Discrete will engage automatically when flags dictate, and that rear surround will go out to the patio. If you disable the rear speakers, it would prevent that, but what will the 7.1 Direct input (which bypasses the DSP) do at that point? I have no idea.

If this works, it'd be a slick way to solve your problem without having to add an additional box to your system. If it doesn't work, I wonder what speakers you have outside - relatively efficient might be able to work with something inexpensive like the little Sonic Impact T-Amp (especially since you only have one source you want to pipe out there).