Depressing.

I've actually contemplated doing the same thing in my setup (after hearing a lot about coffee grounds being used for blue berries etc), but I've decided against it. I think that you could use it, but honestly I dont really see the benefit of it? Everything that I've read says that people use coffee grounds for the added acidity and because it releases nitrogen as it decomposes. I think adding coffee to your water reservoir would simply drop the PH, but it wouldnt be as effective for nutrients as a fertilizer solution would be.

I have heard from a few sources that used coffee grounds are actually closer to ph neutral. Another issue is adding organic material to the reservoir could cause an over growth of bacteria maybe this would be good if you had good bacteria in the res but if there was anything bad in there I would fear for the worst :)