Create curve on existing solid polysurface

Hello, how would one go about creating a closed curve (or just drawing polylines in general) on an already pre-existing solid polysurface which is oriented at 30 degrees?

This is what custom CPlanes are for. If the surface in question is planar, your best bet would be to use CPlane>Surface. You could also sub-object select the surface and use CPlane>Object, but the orientation of the custom CPlane axes may not be what you want. For the utmost control, use CPlane>3Points (Origin, X axis, Y axis).

Once you have that CPlane set up, you can store it with _NamedCPlane and recall it for later use.