Sheldon's improvised drop bolt made from reflector brackets implies your idea is both practical and probably a lot stronger. Make the vertical parts of your bracket as short as possible and use the heaviest gauge metal (preferably steel, not Al) you can manage.
BTW, there are "extra long reach" brakes still out there. Some older bikes used either center pull or side pull brakes with reaches as long as 70 mm. Searching E-bay or an older LBS may locate a pair.