You can always remove the rubber grommet at the back of the brake plate and use a large screwdriver to unscrew the brake adjuster manually ...mmm..it is always a guess(at least for me) which way , up or down...will release...but even as a trial and error method, it will work...
..I understand the screw was broken in the "work" hole, not the "park" hole...right?...
And as you torque the screws to walk the drum, mastering the hammer is a great help... <img src="/forums/images/graemlins/smile.gif" alt="" />