Forgive me but I am a little confused on some of your details but I think I get what you are saying.
I think what you are saying is you want the A button to start the game from the game list correct ?
The details I am a bit lost on are some of your controller bindings. The default controls are A to select through the menus, B is to go back, X will start the game from the game list and Y shows you the box and other art work for t he game.
As for changing this you can remap the buttons in the Controller settings under Options but you cannot change what the functions do and @Jason Carr would have to rework this. A more robust controller functionality is something on the Small Things List and actually got asked for by a few people so hopefully Jason tackles it.