UPnP just cannot be used for synchronized playback on many devices. I guess it could with some non-trivial extension to the spec.
Apropos extensions to the spec, did you look at the UPnP AV MediaServer specifications versions 2,3,4 and MediaRenderer specifications versions 2 & 3? See
http://upnp.org/sdcps-and-certification/standards/sdcps/ (in particular AVTransport:3). These certainly are "non-trivial extensions", and many relate to synced playback functions. To me some of the sync actions seem to be so software heavy that one could not imagine an average consumer device being capable of handling it (e.g. syncing by connecting all devices to a remote master clock server). However the specs are written by people from companies like HP, Intel, LG, Microsoft, Toshiba, Sony, Panasonic, Samsung, Pioneer, Philips and Nokia, so presumably these guys know a thing or two about what future devices will be capable of doing.
Note that although the spec writers are already working on the next UPnP AV specifications above version 3/4, most actual devices are still only on version 1. But don't worry, as anyone can see, the technology of consumer devices is transforming amazingly fast, and I have no doubt that we will see UPnP AV version 3/4/5 devices rather soon...