How to boolean with non manifold geometry? (note : my understanding of manifold is that the object is solid, closed. Tell me if I'm wrong)

I'm working in an industry. I use Blender to adapt ingeniery/simplify CAD 3D models of a power plant for making virtual tour. Part of this work is about making a cut in the plant (like a cake you cut a part to see the inside). 

The 3D models are very complex and requires a lot of work to be manifold (because the CAD software we use makes ugly 3D, with disconected faces and the vertices rarely are where they should be). I don't have the time to make all of them manifold (a power plant is a lot of machines...).

If no error of me, Blender Boolean requires Manifold geometry, and I can't always have it. So to boolean-cut I'm stuck in using 3DSmax where there is the « Boolean » tool, which accept non-manifold geometry (edit: but just cut, it doesn't create new faces). 

It's a pain to switch between 2 3D softwares, and I would like to continue using Blender. Is there a way in blender to cut non-manifold geometry?
I will demonstrate my approach on Suzanne so that it is a bit more complex than say, a cube.


Setup the "knife cutter". For this I use a simple mesh with 3 vertices. Notice that the cursor is set to the pivot point for easy rotation control.





Perform the knife project.





Clean up the cut out vertices. Notes: use wireframe mode so the selection goes "through" the mesh, don't select the verts along the cut line...just the ones inside.