I finally brought my development N900 to the office to see if the Zhaan would play nice with Sonos. My initial efforts were thwarted and alas Sonos did not appear as either a source or a renderer. A quick glance at gupnp-universal-cp knocked me over the head with why — Sonos uses nested devices; something I had not previously tested Zhaan with.
A three line python hotfix later and Sonos now works wonderfully with Zhaan :). They curiously use REL_TIME instead of ABS_TIME for seeking and there is something funny with their implementation of GetVolume that Zhaan isn’t 100% happy with (SetVolume and Seek do work though) but on the whole definitely a success!
Update: A quick apt-get install mediatomb later (and a quick reconfig — mediatomb auto-bound to my VPN interface instead of my LAN, preventing anybody but my machine and the VPN machines, which my phone is not, from seeing it) and my desktop machine is now a perfectly good (and tested) sonos media source. Nifty.