According to StephanieM on the Snapstream media forum at
http://forums.snapstream.com/vb/snapstream-discussion/38482-mce-remote-custom-buttons.html, "For the "Power" button of the remote, by default it will put your computer into standby. But you can disable this in the Windows Control Panel (Performance and Maintenance / Power Options / Advanced Tab and for "When I press the sleep button" choose "Do nothing". This way you'll be able to customize the "Power" button to close BTV if you want."
Not sure if that method will also disable the sleep button on your computer's kbd, or if it will interfere with an other events you may have where XLobby "tells" your computer to go to sleep using the Windows "sleep" command, but if you care to try, once you "tell windows to do nothing upon pressing the sleep button" the process is the same as creating any other IR trigger as long as the MCE remote's power button is "learned" into XLobby's list of devices.
Let me know how it works out.