It can be done - confirmed working
This can be done if you have a NAS that has a web interface for sharing your video content. I have a western digital my world NAS which has twonky media server built in, you can access the web interface in safari on the iPad, start your video playing on it and then AirPlay it to the tv, all without using iTunes.
Most recent NAS devices provide this functionality, and there are quite a few which use twonky, which I can confirm works.
However I personally think jail breaking plus xbmc is a much better way to connect to a NAS