From your question, it sounds like you are looking for a UPnP-capable backend for streaming FLAC to some unspecified frontend(s) for playback. It also sounds like you're seeking a Network Audio Server "NAS", not a Network Accessible Storage "NAS". This is an important distinction to recognize, because the latter NAS is the traditional usage, and most NAS products are mainly file-servers, not media servers.
The difference, really, is hardware vs. software. Network storage "NAS" is a hardware problem with hardware solutions. Network audio server "NAS" is a software problem with software solutions. I think what you're asking for is a product that provides both.
Frankly, I don't know the product space well enough to suggest anything in particular. There are lots of frontends that claim to support streaming FLAC, either from servers in the same product line or from general network storage "NAS" devices, but not many network audio server products specify streaming FLAC and supporting UPnP. Note that most claims of "streaming FLAC" are just marketing-speak for "plays FLAC files from network share".
Any network storage "NAS" devices will support FLAC -- all they're doing is sharing files. UPnP+FLAC is another matter; since the DLNA standards don't require FLAC, it is up to the hardware manufacturer to provide or the end user to add on support. Any hardware NAS product that runs Windows Home Server (or even FreeNAS, as Molly suggests) can run network audio "NAS" software as an add-on.
Escient's frontend products claim to stream FLAC from their backend products, but I've no idea whether their backends work with non-Escient products, or whether they support UPnP.
Acer's easyStore H340 is a network storage "NAS" that runs Windows Home Server on an Atom230 processor. It includes DLNA compatible digital media server software, and the documentation claims FLAC support, but I don't know if this is a WHS built-in feature, or an Acer add-on. I've considered purchasing this box and generally like the design and features, but frankly I'm a DIY-er and would rather build my own Atom330-and-Linux-powered version.
Asset UPnP is a software network audio "NAS" that runs on Windows (including Windows Home Server). It's DLNA compatible and serves FLAC and many other formats. The server functionality is free, but Internet Radio and other features are only available in the registered version.
Logitech's Squeezebox and Transporter devices are frontends, and you install their Squeezebox Server software on a PC or network storage "NAS" as a backend. These products support FLAC and transcoding but don't do UPnP (to my knowledge).
FLAC's homepage and Hardware links page is a good source of information on FLAC-capable products. See also the Hardware support section of Wikipedia's FLAC article. Most of these will be frontend products capable of decoding FLAC formats, but some manufacturers may include backends in their product lines.
Realistically, I don't think anything will be fully plug-n-play in the way that you want. But a Windows Home Server product, possibly with add-on media server software like Asset UPnP (or even XBMC/TVersity/Boxee or similar), may fit your bill for a NAS+NAS backend. I hope some of this info helps.
There are many supplied answers. Please check which is your accepted answer. – Johan Karlsson – 2012-02-06T15:36:14.060