I think you're looking at it in a less optimal way. You shouldn't need software on your server to make it work. As long as it's using standard protocols like Samba for network file sharing, then you use that instead adding in an additional piece of software to talk to. It's just like how XBMC doesn't need software at the other end for it to stream media. You just point it to the source and it can grab from there. In the case of an app on your phone or tablet, all you do is have it find the source through open standards like Samba, and then that gets pushed to the Chromecast as the source and it can read from there at that point.
Well I'm not sure how much less running a samba server on your NAS is than running a DLNA one or a Plex one.
The issue in all this though is unless all your media is in the exact formats the ChromeCast device can handle it needs some processing beforehand. Certainly that a major part of Plex, the other part being it manages a fancy interface with tons of metadata. And I'll admit thats a part that makes me like it over a pure DLNA system with boring filelists and such that you get with a DLNA setup on last gen consoles.