I like your idea for the Northeast Dory and have been wondering when someone would do this. I think you would have to add a boomkin through the transom to handle the mizzen sheet. I also wonder how the tiller would operate with the mizen placement. On many of these small yawls the mizzen is placed directly on the transom - not easy on the dory. Also the center of effort would change to the rear and the standard sail placement might have to be modified. The advantages of having a mizzen are well known such as during reefing and raising the main. I think it might work out well (correctly calculated which is beyond my level) and if you make this modification give us lots of pictures of your progress.