The Super DC-3s had disks from Douglas, I guess it wouldn't be too hard to get an STC.
I had disc brakes on my old DC-3, I believe it was called the 'Convair Brake Conversion'. On initial application the disc brakes did not seem to slow the aircraft as rapidly as the tube brakes, however, you never had the problem of the brakes fading either.
You can't really tell from this picture, however, I never thought I'd need a picture of the brakes on the DC-3.
By the way, we didn't fly it that much and I had a mechanic (engineer) that just loved working on the old gal. He kept the oil leaks to a minimum. We ran the engines at least once a week and flew it once a month, even if it was just around the pattern. Fortunately the owner understood that we couldn't let it sit around and let me fly it when we needed to. It was fun to take to local airshows in the summer.
Sadly she is sitting all but abandon at some airport in California. The last picture of her that was sent to me she was looking in rather poor condition.