I am posting in hopes on finding a method of creating a 3D routing-offset tool path.

Specifically, I am trying to part a vacuum formed plastic component from the surrounding material. This part is complex, and the edge I want to cut along is 3D, (picture a roller coaster path).
I know how to machine a 3D surface, but creating a single line path that follows a 3D surface eludes me. Apply to Relief doesn't seem to have any effect on the Routing-Offset tool path, nor to an Engraving toolpath. I've tried creating a narrow relief that follows the perimeter, but when I create an Island Fill tool path, it comes out very jagged. I feel like I am close, but can't quite figure it out. Also, the way I am potentially going to get it to work takes a lot of steps and is convoluted, so finding a simpler, quicker way would be great.

If anyone has expereince with reliefs and 3D tool paths, please chime in.

Thanks!