Further to my earlier post: https://tickett.wordpress.com/2012/01/21/sonos-up-and-runnings-ish/ I have now received the Sonos Connect (ZP90) and have now solved issue #1 (stream to both Sonos S5 / Play 5 devices and my living room Onkyo TX-SR608 amp simultaneously) but issue #2 is still outstanding (streaming from the Onkyo TX-SR608 to the Sonos devices).
The problem is the lack of analog audio outputs on the Onkyo amp. Strictly speaking there are 2 sets of analog outputs on the amp: Zone 2 and VCR/DVR but they only work if the input is analogue (which in most cases it is not).
As the main device I’d want to send audio from the amp to the Sonos devices is my AppleTV 2 I think I will probably buy a Digital Optical to Analog RCA audio convertor (£20 off eBay)- this is also the more energy efficient solution as the amp won’t have to be powered to feed the ZP90. Unfortunately this will leave me unable to stream audio from the sky box, htpc etc but it’s unlikely I’d ever want to do that anyway…