i added two drops of mint extract (the clear kind from the grocery store baking isle) to a pint of stout and am drinking it right now. The stout isnt great, but the mint is kind of nice. It's alot stronger in the aroma then the flavour.
My plan is to make a chocolate stout, add cocoa nibs, and some mint leaves in the secondary, and perhaps add some mint extract at packaging. The extract would be grea aroma, the leaves would give flavour, everybody wins.