Figured out that a Copy Rotation on his Torso Pivot, aimed at the Root, with XY inverted and the Influence at about half keeps him upright on those inclines. Neat little trick to get some extra value out of a single walk cycle.
Fooling around with the Shrinkwrap constraint. Root bone constrained to an object in Project mode + Aligned to Normal lets him stick to a surface. Maybe not very practical, but it's neat.