Thank you very much @mpoole32 for your detailed feedbacks !
The main goal of this feature is to export a useful DXF file for CNC owners with the minimal of click. But not to create G-Code directly in SketchUp. Because target user of OpenCutList is larger.
I don’t want to go to Simple DXF approach that consists of editing the model in SketchUp to integrate DXF meta data.
Currently, my code analyzes the geometry and export a 2D projection. Then it know what polygon is at what depth.
The question is what is the best way to keep this kind of data in DXF ?
I can create a BLOC for each part, that contains a BLOC for each depth (maybe better to use layers for depth … 
?) :
example.skp (33,3 Ko)
example.dxf (12,2 Ko)
@napperkt could this hierarchical structure suit your needs?