A bit late here as I have been pretty busy with other stuff, but here’s a slightly different variation - also a Python script, so it will run on Mac. It will select either objects strictly co-planar with a given planar curve or planar surface, or any planar objects on a plane parallel to the selected object - there is a command line toggle between the two if no object is preselected. The set option is sticky within any given Rhino session - defaults to strictly co-planar on first run in a new Rhino instance. Works with a pre-selected surface or curve, will ignore non-planar objects or multiple pre-selections. Selects only the type of object first selected - if a curve is selected, only curves are considered, if a surface is selected, only surfaces are considered.
Edit - updated version, the previous one had a small selection bug. @MisterB
SelParallelToPlanarObj.py (4.6 KB)