Thanks for the question. This is actually rather tricky to achieve in RailClone because it is designed to create objects using rows and columns and this requires something more like a random scatter or a packing algorithm. Especially because it seems a cylinder should be placed where each ring intersects, which RailClone cannot detect.
Nonetheless I attach a file which achieves something similar by randomising the rings and their sizes and then uses negative padding values to get them to overlap. To create a "scattered" effect I've randomised the vertical transform value.
To create the area I have rotated the RC object 90 degrees so that it's Z axis points towards the surface
. You can then extract the surface's boundary spline and use it as a clipping area
. As RailClone may give artefacts when the clipping spline is the same size as the surface, I've extended the surface's boundary slightly.
Please take a look at the attached file, and please let me know if there's anything else I can do to help out.
ps. If you want to use Forest then you may find this trick using VRayDistanceTex
works and has the benefit of maintaining instancing.