Yeah, batten with a wrench taped to it (thoroughly). Tape the bolt and washer on an open end wrench being a little careful to not put tape too much under the bolt/washer. If you have the time, silicone the bolt and washer on the wrench the night before (I would prep two).

I don't know how hard it's going to be to get around the internal castings in the beam ... I suppose it's possible that you may need to remove those (I've never dne that).

Be glad that you found this now. That could have cost you a beam and left you dismasted in a rather unpleasant situation.