Putting a potentiometer on the Water temp sensor will ONLY make it richer, since it can only ADD resistance. HOWEVER. if you take off the plug going to the water temp sensor and use the potentiometer in place of the water temp sensor, as long as it has the correct range, then yes, you can tune it to be leaner or richer than what it normally would be. And I have no idea if you could do that to the AFM. I really doubt it though.
So basically, deleting the water temp sensor and using a potentiometer would do exactly what you want to, without touching the stock AFM. of course, you'd be wise to have a air/fuel gauge.