as a person who has worked as a barista in a number of cafes for a number of years the op tickles my funny bone. i have definitely done some interesting things attempting to brew coffee, so i can relate. although i am thankful i have never poured anything other than water
through a reservoir.
acting on a curious impulse early on in my barista career i added blueberry syrup to a mild coffee, then to a mild coffee ground and run
through the espresso machine. that was obviously not its intended use. if you want to experience something vile, i once tried a lime-coffee recipe. yuck. even worse than my chocolate chai experiment. i know some people really dig it, but it's over my head. i tried and tried different variations and approaches of the above, and nothing was palatable.
<3