[GeeTwo]

I don't know how you're reading the "blue box". It specifically allows non-horizontal bumpers, as long as they stay in the 2"-10" range. Since the bumper is specified to be 5" high, that lets you slope it a massive 3". Even over an 8" run, I don't see that sloping it at atan(.375) =20 degrees is an "overt deviation", but then I'm not a judge. I think that they're trying to keep you from doing vertical pieces or something else silly, like putting a third row of pool noodle into that 3" of space.
That said, if you're design requires a bumper (or any other part) that you can't be sure fits the rules, change the design, or at least make a backup plan!