I recently added lactose (boiled and cooled), as well as some other ingredients, to enhance the flavor and mouthfeel to a keg of about 3 gallons of pumpkin ale that had been on tap for a couple of months. Seemed to work fine and I considered it to be a success, as far as achieving what I was hoping for. One thing I did, however, was to put the lactose and other ingredients into a fresh keg and then CO2 transferred the beer from the old keg to the new. I did this in order to mix it all in without having to worry about kicking up all the yeast slurry that I knew was sitting at the bottom of the keg and clouding up my clear beer. It was still crystal clear immediately after transferring.