FIGURED IT OUT! Here's a solution to this problem for anyone who might find this post in the future...
Basically, you can add play modes in the edit screen. And games can have multiple selected. Sort launchbox by max players, then you can mass select your games, and hit ctrl+E to mass edit all of them and add the new mode.
That way, you can easily tell Big Box to show just 4-player games or co-op games, etc.
ALSO: Many games already have the "cooperative" play mode named, so all you really need to do is add the max players field to the play mode field and you'll be able to sort