Personally I think its pointless trying to configure xbox to do the job of a comercially available pvr. Too many components to keep running and crappy flexibility.
Now, if you had a RePlay TV pvr, and xbmc hooked to your xbox, then you could use your xbox to stream movies recored on your RePlay TV. Heres the advantage.
Lets say your RePlay TV is in your Bedroom and your Xbox is in your livingroom, and both are connected to the same network. Then you could watch all your Replay TV recorded movies in your living room from your xbox. Its like having two pvr's, but the xbox is play only.