This is nice, though I think the use of indices instead of stable identifiers might bite in complex models that undergo changes. I've been toying with the idea that you could specify a point (2D or 3D in some coordinate system, depending on context) and pick the face/edge/point closest to that as the identifier. The only other alternative I see is diffing old and new, and trying to match the outputs of each operation geometrically, but that would produce extra output that needs to be persisted...
E.g. when splitting a face in OnShape, I might have to redo a whole bunch of operations later because the identifiers change, but I'm often surprised how good it is at matching up faces after a single operation. Like modifying a sketch, then having to add the new face to an extrusion, but then it magically does the right thing for chamfers and drafts.
Yep, topological naming solving is a hard problem to solve. It took FreeCAD many years to get it to work correctly. That's why I went with indices for this version. You can also use filters like extrusion.endFaces(face().arc()) to get only arc edges.
Regarding the picking; that's exactly the region picking feature in this screenshot: https://fluidcad.io/img/region-extrude.gif
When the user click on a region we insert a 2D point, then the extrude feature will find the nearest face. I have experimented with adding this picking logic to selections too select().pick() which will probably be merged in future releases.
Nice!