Found a nice way to make this a one man job. Use a hack saw to cut a notch into a nut of the same size as the one on your bolt. From below put this nut onto the bolt and clamp it with vise grips. this will keep the bolt from spinning. you can then hold the bolt still, and twist the holding nut with another wrench. works nicely.
I'm not sure I really followed you on the notch thing, but I've definitely used vice grips to hold nuts while I spin the bolt. Generally there is something in the way to keep the vice grips from turning...
Now you only have to climb into that dark little corner 4 times per stanchion!
The best is a buddy sitting on your deck with a screwdriver in one hand and the beer you supplied him in the other.