Well, if your "NAS" is a computer running iTunes, connected to your network, I know the Music app on an i-device can access / play "shared" iTunes libraries. So if you do that and control the i-device directly, it could serve as a music transport. My personal experience is that the Music app does take a long time to make the initial access to the shared music library if that library is large (as in my case).
The problem becomes if you want to then use a third device as a remote control. I-devices have the free "Remote" app to control iTunes running on a computer. But this will control iTunes on the computer only - it won't make the iPad / iPod as music transport receive the signal. And Airplay in iTunes doesn't include i-devices (iPad / iPod), just apply tv's, airports, and things such as receivers that have airplay built in.
That's the limit of my experience. The only other thing that occurs to me is that there may be other software out there for i-devices that allow them to access / play tracks on a NAS. I still think you'd run into the same problem as above though even if you had two i-devices with the same software. I.e. that using one device to select / play the track doesn't mean it will be output to the second i-device.
In the end I think it will be easier & probably cheaper to use existing solutions, i.e a computer running iTunes and an apple airport express or apple tv along with an i-device as remote control. An AES or ATV costs a lot less than an iPad + dock, and the former (computer running iTunes) maybe something you already have (or can be bought for less than an iPad).
If you don't want to use Apple hardware, a used squeezebox should work well with an NAS. I know some NAS's even used to support squeezeboxes directly. Can't remember the specific models though.
Hope that helps.
