I'm updating my answer with a more clear step-by-step explanation since I just ran across the problem again and I couldn't *quite* remember exactly how I fixed this the first time. So I documented this for myself and I thought it would be nice to share. It's a bit of a painful workaround if you have a custom CPlane, but it works for me.
FYI, I'm using a 5-axis machine for wax cutting (jewelry), so my axis of revolution is in the Z-axis and the solution below is based on that orientation:
A. To reverse the direction of a toolpath:
1. _Mirror (with the Copy flag set to yes) the surfaces, polysurfaces, and/ or meshes (and boundary curves if applicable) being selected for tool path calculation in the world Z-axis (front camera).
2. Generate the toolpaths.
3. _Mirror the toolpaths in either the X or Y axis (i.e. front or right camera, Copy flag in the _Mirror command set to no).
4. _Mirror again in the Z-axis (i.e.front camera, Copy flag set to no).
5. _Rotate 180 degrees in the Z-axis (i.e.top view).
B. If one is generating the toolpaths from a custom CPlane (for example, in order to use 5-axis From Boundary Curves), the custom CPlane will need to be mirrored as well in order to correctly generate the toolpaths:
1. Draw a plane (_Plane) in the viewport set with the custom C-Plane you wish to mirror.
2. _Mirror the plane in the world Z-direction (front camera, Copy flag set to no)
3. Create a new viewport (_NewViewport or Viewport Menu > Viewport Layout > New Viewport).
4. Name the viewport in Named Views to something clever (_NamedView or Viewport Menu > Set View > Named Views...)
5. Use _OrientCameraToSrf (in the Set View toolbar, has a camera icon) to set the CPlane to the plane you created (pay attention to the surface direction and use the flip flag as necessary to make sure the camera is oriented the correct direction).
6. Use the _CPlane command and use the View flag to 'Set CPlane to view' (CPlane menu, icon with an eye).
7. Delete the plane.
8. From here you can follow the steps in Section A above to mirror the source (and boundary curves if applicable).
9. After you're done, you can close the viewport (_CloseViewport or Viewport Menu > Close Viewport) and delete your duplicated/ mirrored geometry and curves.