We’ve chatted about this internally and I want to make sure that we clearly understand all the workflow issues before we do any typing.
You can select a collection of SubD vertices/edges/faces and Align > ToPlane will project the associated control points to a plane. (If a face is highlighted, all of its control points are projected and if an edge is highlighted, both of its control points are projected.)
There are a variety of ways to select SubD components - various “loop” tools, subobject select (Ctrl+Shift+click).
Is it the case that the subobject selection part of the workflow is good enough to do what you need?
If not, what is not working right with subobject selection?
Is it the case that Align>ToPlane this would do what you want if you could easily specify the plane (say using 3 point plane as you suggested)?
If it’s not the case, can you describe what Align>ToPlane is not doing that you need to solve your workflow issue?
If subobject selection and Align>ToPlane do what you want, then it seems the thing we need to focus on is a general issue that the ToPlane option doesn’t make it easy to get the plane you want/ Is that correct?