Surely you must be thinking of The Spector Effect. How about a button to make everything sound like it comes out of a Ghetto-Blaster? That's The Eno Effect, if you haven't guessed.
Go back to sub-mixing everything through the earlier mixers. You'll eventually get left with one Mixer Channel. Now look closer at what's there. Take off the spread, and, Hey Presto! Mono!
Your wish is already catered for!