There a re a few places on the web (e.g., The Green Button, Media Center Sandbox Blog, driverheaven.net, etc.) that discussed various way how to launch Media Center and direct it to particular applications. Two most command ways - command line arguments (directmedia and homepage switches) or sending a Win32 message to the MCE window with certain wParam and lParam settings.
I have read all the info and played with it myself. Too many limitations there. Basically you can only redirect the MCE to some [very few] applications - to Live TV, Video Library, etc..
A hacky way I tried - to grab the XML resource names off the MCE binaries - didn't work well. Most of the XML pages there require some machine state variables being passed from the main application entrypoint and this makes the MCE unstable. Mostly it just crashes at the launch.
Only helpful XML page I found I could bring up that way, that wasn't mentioned on the Web, was the Media Center Setup page. All you have to do is to launch MCE with the following command line:
%SystemRoot%\ehome\ehshell.exe /homepage:Options.SetupMediaCenter.xml /PushStartPage:True