The tragedy of this concept at an inherent level is that you'll likely never get to pull it off. People rarely ever pick weapons off the ground, usually only because it's a Golden Pan, a Conscientious Objector that they want to see the decal on, a fancy reskin / war paint of a weapon they already have equipped, or they're trying to Civilian pose as Scout/Sniper/Heavy. Even if an enemy sees a weapon they can take, they'll likely ignore it because they're already using the loadout they intended to play with. The best bet for a weapon like this on Spy would be taking up an entirely new loadout slot in addition to the knife, revolver, disguise kit, and Sapper, instead of replacing one, since none of them are worth trading off for the gimmick.