I am working on this and have something that works in Rhino 6 on a Windows machine 99% of the time on my large meshes with 18M faces where Rhino’s Mesh Tools -> Trim Mesh fails over 90% of the time. I am in the last stage of fixing it up for the few remaining known issues I have identified. It works best on triangular meshes but I am starting to get it working better on mixed triangular/quadrangular meshes. It is a Python script along with a DLL (optional) for speeding up volume/area calculations. I packaged it in an Eto GUI:
It uses a closed line as the trimming boundary. Currently you cannot cut outside the boundary of the mesh but this could be fixed. You can enter points for the boundary line, select an existing line or reuse the previous line stored on the Corners layer.
Here is a cutout from a 18M face mesh:
This mesh has 18M faces as illustrated by this wireframe-view closeup of cutout above:
The main procedures in the script are shown below:
The script moves your original mesh onto the Start Layer and uses other layers to save inputs (the boundary curve) and results (the trimmed mesh):
I would like to try this out on one of your hardest to trim meshes. If you could send me a link to the .obj file for the mesh then I could make sure it works and then release the Python script (about 3600 lines with over 50% comments). It would also be helpful to know exactly what type of trimming operations you need to perform. Do you want to trim, like in my example above, or make a hole or make a trench (with sides and bottom filled in)?