had a similar idea myself as I too use an old xbox with xbmc and coinops, my plan was to intergrate a pi in the xbox housing just for playing the higher res movies as you prob know the xbox aint great with mkv. I planned to use the power cables going to the controller ports for power to the pi as these are more or less general usb standard but I was struggling with bringing the outputs on the pi to the back of the xbox case (hdmi, Ethernet , sd-card, usb) . then there was the issue of the 2 devices interfering with each other . in the end I scrapped the idea as it seemed too much hassle and now use the 2 devices independently of each other .
Originally hoped to intergrate it into the "slim clock xbox" for my daughters bedroom, but space is a bit tight it there
pic attached
http://imageshack.us/photo/my-images/853/p1050081s.jpg/