RE: SUP on an old longboard, my experience..
"Poling" with a one-piece mast also works. Several years ago at a windsurfiing event on a day with almost no wind I joined in the fun others were having poling longboards using one-piece masts.
Using a one-piece mast for poling a board can put as much or more poling surface area in the water as a paddle would because the mast is so much longer. Poling with the mast tip on one side and the mast base end on the other side makes the rythmn smoother. To equalize the steering effect you can either put the tip end in deeper than the mast base end or just do more steering with the mast base end.
As a kid in geography class decades ago when I read about people who poled dug-out canoes through swamps while standing up I thought they used the poles to push off the bottom. At that time I didn't think of the mud at the bottom that would make that unworkable so I didn't understand poling at all until I tried it just a few years ago at that windsurfing event.