Thanks for the interesting question. As you've pointed out, in Path mode, it's not possible to use normal exclude areas. Instead though, you could do something like this.
- Duplicate your source spline as a reference
- Create another spline object where you want the access areas to appear
- Use the Shape Boolean object to cut away the parts of the references spline you don't want. Add it to the Shape Boolean as a Subtract area in Cookie cutter more. This will of course remain parametric.
- Use this new spline as the path for Forest Pack. Unfortunately the boolean object flattens the splines on the Z axis. If you have a stand, this is an easy fix - just add the stand to the Forest Pack surface list.
- If you don't have a stand that's suitable, then you can duplicate the original spline another time - again as a reference.
- Add a Renderable Spline modifier to the new spline and add an Edit Poly modifier above that. Now use this as the surface instead. Turn on Auto Update in the surfaces rollout if you intend to make changes to the distribution.
- The whole setup is parametric, you can modify the original spline at any point and it should all update.
A sample is attached. Because this is an interesting question, I'll try to include it in tomorrow's live stream.