NaroCAD is compartmentalized in more parts to make things replaceable and extensible. One of the components that you use for drawing custom shapes is named FacePicker, that will detect the plane you are on. So if you draw your circle, it is always drawn on the last plane you're mouse was on.
So if you will click on a shape that is one face (like rectangle, circle, ellipse, or a generated face), the floating toolbar will show sketch shapes, and those shapes will be locked to the plane your face is pointing. This need more changes, and I've tried to make them to not be invasive. The single change that is needed in a meta-action to be "plane-locked" is to write Dependency.LockPlaneIfRequested = true; and to be added in the sketch tools.
For locking a plane, you have to write something like Inputs[InputNames.RestrictedPlane].Notify(NotificationNames.SetPlane,
Those changes are visible in the nightly build.