Hi Peter,
Because new_core_editors.use_new_problem_editor is a WaffleFlag, not a CourseWaffleFlag (in the code) it cannot be overriden with course overrides.
At edx.org, we partially rolled out the experience by enabling the flag for a particular group. Disabling it for a particular group would require: enabling it for all other users except those you want it off for…
You could certainly make a code change in edx-platform to make the flag a CourseWaffleFlag here https://github.com/openedx/edx-platform/blob/ee871730c8edff953373b3e5b5e97e566b9af66c/cms/djangoapps/contentstore/toggles.py#L143. If that is something instance owners want, I would approve that PR.
Not enough… GitHub - openedx/edx-toggles: Library and utilities for feature toggles is my source of truth.