I read this to the only game i remember it works for is metal gear solid 3 cant remember the name of the others.There is actually. I remember seeing a tutorial, but it only worked for like four games. What you had to do is:
Download the Demo to those games
Load of MW2
Start up a Private Match
While the Private Match is loading, press the Menu button
And go to "Play (Insert Demo Name Here) Demo"
Sorry, I can't remember the names of the games, but if I find it; I'll let you know via PM or I'll post it here.