Isn't this what Kodi (formerly XBMC and source from which Plex forked) is all about?
Kodi also has an optional web interface, which is primarily used to act as a remote, but I've found it can also be a thin client receiver--any media within the library can be streamed to the browser tab.
This can be combined with a UPnP approach if there are also non-dedicated devices that need to access media.
[0] - openelec.tv
I am running the plex client on dumb devices like tv's. That gives me a nice UI, sync of watched locations and automatic transcoding of content that the client cannot play natively.