While you don't have network filesystem support, that doesn't stop you from loading that same content over HTTP. Think of TV Widgets as DHTML / Web 2.0 apps which instead of running in the browser instead run on your TV in a constrained (performance wise compared to the desktop) environment.
Now, back to your idea. What on that network attached storage would you want to access? Things like word documents and random files doesn't make sense (since the TV doesn't support those formats). As to pictures, music, and videos (which do make sense) there are several options:
1) Many NAS devices have an HTTP interface to them. Some of them even have a web API. If your NAS device support this, then it's a simple matter to interface it via the normal XHR mechanisms.
2) If your NAS device isn't endowed that way, then you can use a PC on the network. This might even make more sense because you can have an application on the PC which indexes the data.
As to #2, I have developed just such a setup for my own home. I have a Mac Mini which has Apache turned on. I wrote a basic webapp which indexes all of the media content sitting on my NAS and provides a REST based JSON feed into that. The widget then uses that API to search for content and then displays it on the TV directly streaming it over the network via HTTP. The widget itself is maybe 250 lines of Javascript code in all and handles pictures, videos, and music. Pretty simple to do in fact and works great for my personal needs.
