I’m happy to do some testing for you, although my requirement for UPnP is solely because of the non-iTunes use of Windows XP.
My personal streaming requirements are at the moment only to my linux laptop and maybe later this year a Roku Soundbridge, finances permitting. On my linux desktop I simply listen to my locally stored files, which I backup to the Slug, which in turn has Firefly installed to stream to my laptop and in the future a Soundbridge,
I’ll have a go at running both Firefly and Twonky or some other UPnP server on the Slug and see how it handles it. The load at the moment is minimal as it’s only running Firefly and CUPS+Samba and it seems that people have running quite a lot of services on a single Slug.
I have subscribed to the feed and will keep an eye on UPnP development.