Thanks for the tip, Jan-Ruben, but I think that method is for determining if a point is contained within a closed brep... as in within its interior. I'm looking to determine if a point which lies on a surface is within the trim boundary or not.
Surely this functionality is around somewhere - the inbuilt divide surface component in grasshopper already does this, but I can't reproduce the behavior within a script node.
Got it... I think.
It involved a bit of digging around in the Brep class, and some education on how trim boundaries are handled. Check out the attached ghx file for a cleaned-up function which performs accordingly - important opennurbs functionality includes: OnBrep.ComputeLoopType , OnBrep.Loop2dCurve , and RhUtil.RhinoPointInPlanarClosedCurve
Would be most appreciative for a heads-up for cases where this function would not work!