I just discovered this glitch from reading this article here: Worthplaying | ‘Modern Warfare 2’ (ALL) Bug Grants Free SNK Games
Essentially, with either a demo or full version of CoD: MW2, you can play the following SNK XBLA games in their full unlocked versions.
Garou: Mark of the Wolves, The King of Fighters '98 Ultimate Match, Metal Slug 3, Metal Slug XX and Samurai Showdown II.
I just tried it on Metal Slug XX and it works. It’s the weirdest thing ever. So until a fix comes out, enjoy your time with these ‘extended’ demos. :nunchuck: