Can't you import your existing movies into your itunes library on your PC/mac or whatever? If you can get them into iTunes then any Apple TV will be able to play them in the same way you can play your music. No need to stream from a device.
There are a few software options out there that can convert movies from one format to an iTunes compatible format.
Problem with that is apple TV can't just play them - it needs a mac of some sort to run itunes for the library access. It's the reason why the jailbroken ATV2's are so expensive - it means you don't need another server as you can just run it as a DLNA client.
If I want to play my movies I have to fire the laptop up just to play the apple TV which I find annoying. You can't just stream via the iPad either as most streaming apps don't let you do anything else whilst using airplay so it means you can't browse web etc while watching the film.
OP not sure on Aceplayer but I use a couple of media players on the ipad which do what you're talking about fine, talk to the NAS and stream directly to the apple TV, so it's quite feasible if a little faffy. Not just MP4, pretty sure i've had other non apple formats working.