If I understand you right, you want to do something like this?http://www.mepix.info/varomex/hlo/1512.jpg
A gravity which is goes "through" your plain and nails a marble on the ground?
As far as I know there is no build in function in chipmunk for it (cause its a 2d physic engine
), however you can implement this (fake) gravity by yourself.
In this case (if you assume that the marble always touches the ground and cant "fly") its not very difficult.
For example the board is parallel to the ground, no additional force will take effect on the marble. But if you change the board for example around the Y axis, the marble will roll along the X axis.
So all you have to do is to project the gravity Z axis on the X/Y plane and add the force to the marble. The math behind it is not that difficult (sine / cosine etc).