When I bottled a stout a while back, I added in a few shots of cooled espresso to my bottling bucket. From what I've heard, boiling the coffee will release more bitterness, but I would assume that refers to boiling the coffee grounds, which you didn't do. Is the water in the mash tun hot enough to brew the coffee? I thought coffee was usually brewed with nearly boiling water, which a mash never reaches. But perhaps the cooler temps would bring out the flavors without the bitterness? I guess that when you are getting ready to bottle or keg, you could taste the brew and see if there is enough coffee flavor, and if you want more, just brew some jo and throw it in there -- I have heard of people mixing chocolate in there as well, for more of a mocha effect, yum! I dunno, but I'll be curious to hear how your method works out!