I recently started working on a Rhino.Python script that accesses another CAD application / modeling engine via scripting without leaving Rhino’s GUI. The script is an alternative to _FilletEdge for when Open Cascade, via FreeCAD, can solve some constant radius, rolling-ball problems like those reported to the forum and/or are currently listed as Bugs in YouTrack.
I’m using Rhino 7, 7.13.21348.13001 and FreeCAD 0.019.24267 on Windows.
The following are a couple of examples of _FilletEdge vs. FreeCAD’s Part Fillet. Both FreeCAD/OCCT’s solutions include a patch-like surface at intersection of the edges. (Notice the setback fillets in the last GIF. They were not specified.)
I’m going to work on the script more before publishing it.