Interesting,
I do think a back support is important, I use a 'beach' stool, a very low one, you almost sit on the ground, but you can use your legs as a tripod and because you are so near the ground, you can use the surface around you as a table. It might take about an hour or so to do a watercolour, so a decent set up will help.
Wonder what seat / stool / pillow other outdoor sketchers use?