I don't see any theoretical problems to the mix just don't think your balancing ph, more than your adding lime to your coffee as a additional nutrient. a 50/50 mix could be disastrous, same thing goes with how you apply it, ash on the leaves kills spiders and acute foliar application could really screw up plants. On the flip side if you can apply it onto mulch the ash will
boost the coffee's slug repellancy while dry and slowly at potash as
water leaches it through the soil layers. I can't seem to do anything delicately so i usually throw grounds at the plants in the rain hoping to hit slugs in the face and let the rain wash it down. I figure why put it where slugs might be, put it where there going so each time the climb it falls in their face. I wonder if at the right moisture would coffee help bind up the ash so it's less mobile, or would it just destroy the bacteria that the worms are actually feeding on when they go at coffee grounds.