Data sheet for older Xbox 360 but will work with slim, gives the pin outs for the audio/video. You can use that to wire in whatever you would like VGA or add internal screen. The data sheet is pretty detailed it tells you which pins to bridge to tell the Xbox a standard screen is connected. Just wire a spst switch between those pins, for audio jack left and right ground are solderd together to the ground of audio jack.
I remember seeing a tutorial out there once for installing a screen but I can't find it now, if someone doesn't beat me to it I may do one.