Im sure it is posable to sniff the packets sent to your 360 with XNA and then resend them with different files. Even though this sound like a good idea(send a different program or game) its just not that simple.
I havnt been keeping up with the scene much so I might not be to acurate but this is just my opinion on what I think something like the XNA software is doing.
First it builds your program or game on your PC, The executable files that it creates somehow get stamped as XNA.
Then the computer talks to the xbox360 and says, hey im going to send over "Name of your game".
The xbox reply's, "OK, I'm ready to play some games

".
After your PC transfers the files, the xbox checks the executable to see if its the correct game and if it has been stamped(signed) as an official XNA executable.
So in the end if you can only run the Signed xna files, why not just transfer them like your suppoed to?