The MPOLYGON seems to be an AutoCAD Map specific entity but very similar to the HATCH entity and is has no explicit documentation in the DXF Reference. I can not create this entity in BricsCAD, but the MPOLYGON could be supported if I get some test data from AutoCAD Map users to see if the entity is really like a HATCH entity. I would need small DXF files containing MPOLYGONS with polyline paths (LWPOLYLINE) and edge paths (ARC, ELLIPSE, SPLINE), with and without nested holes, with and without filling in many DXF versions (R2000, R2007, R2010, R2018).
A new feature will be the conversion from MPOLYGON to HATCH and the HATCH entity will get a method to extract the bounding paths as virtual as DXF primitives:
If you do not want to publish the documents publicly, you can send them to ez...@mozman.at. The files are treated confidentially.
The main thread at github: https://github.com/mozman/ezdxf/discussions/411