How can I mount a composite rail kit on a ramp shallower than the brackets accommodate?
For that shallow of a slope I'd be inclined to look at level railing kits and their associated hardware instead. That would be a much closer starting point. I'd then mill some 11&deg; wedges from white vinyl to place behind the brackets, tilting them into position. 

If that's not a good solution, create some 20&deg; wedges (31&deg; - 11&deg;). Pre-drill them so that longer mounting screws pass through smoothly at the bracket mounting hole locations. 

Solid vinyl board stock is readily available at home improvement stores. Try to find pieces that allow you to leave the factory milled finish on exposed faces. Cut faces will show a rough, porous surface.

Ideally the wedges will be roughly 1/2" wider and taller than the brackets (for a 1/4" reveal all around), and smaller than the posts (to avoid flush joints, which are tricky and can be unsightly).