Having a brain fart here - I have some RhinoCommon geometry (curves, surfaces that are not in the document), and I need to get a plane-aligned bounding box for each with a plane that is not one of the principal XYZ planes. I need to do this a lot of times.

I have tried with

xform = Rhino.Geometry.Transform.ChangeBasis(Rhino.Geometry.Plane.WorldXY, ref_plane)
#and then
bb=obj.GetBoundingBox(xform) #one single object here

then getting the corner points and transforming them in the other direction, but this still seems to get me a world-aligned bounding boxâ€¦

Hello. I think the whole trick is to realize you are getting a plane aligned boundingBox and this box is defined by coordinates for the Min/Max corners. Now the coordinates are in the plane space so if you want to materialize the box use Rhino.Geometry.Box(plane, boundingBox) where plane is your plane and bounding box your plane aligned boundingBox.