I am not entirely familiar with the internals of the Crystal Space engine itself, but would it be possible to set up (though again, a strong level of complexity) additional gravity point or plane sources, potentially in different directions? If not, would this be worth the time to code?
What I am considering is escherian space, as a puzzle. If you can create planes/spheres/oblate spheroids as part of ramps, stairs, etc, you can achieve interesting effects throughout. Basically at that point the \'normal\' to the character could be perpendicular to the current gravitic plane. I am imagining a point at this moment where you are on the inside of a sphere, the gravitics pointing outward from the sphere, as a quest location.
(which, I admit, is far, far kinder than my 4 dimensional maze I was working on elsewhere.) *smirk*
