All major engine makers in the late 70's - 80's offered inflight mixture control for their engines. I had OS, OPS, Rossi engines with aux mixture control. A lot of marine race engines had the same mix controls. As for a universal mixture control I have never seen one (to many different carbs), but you if you know anyone with a large colection of RCM mags look through as I remember someone offering a inj/mix control for engines.

Cheers