OH right, yes I have seen that before it IS awesome!
This is actually done in Ply's mod, an FPSC mod made by Plystire, which is actually going to be merging with other wonderful mods such as Airmod, Lemur, and s4mod to make one gigantic FPS Creator modification!
Here you can take a look at the merge, called Project Blue UM
http://forum.thegamecreators.com/?m=forum_view&t=149500&b=21
Or you can take a closer look at Ply's Mod specifically, which is what was used to create that decal over-ride.
http://www.plysmod.rgccampaign.net/index.php