Answer
Yes — right-click directly within the specific tab/sub-area you want the Custom Menu scoped to (not a neighboring tab), and create the Custom Menu from there. Each distinct tab or sub-area of a multi-tab screen corresponds to its own distinct underlying 'Window' as far as Custom Menu creation is concerned — right-clicking in a nested sub-tab produces a Custom Menu tied specifically to that sub-tab's Window, right-clicking the parent tab one level up produces a different Window, and right-clicking the overall header produces yet another. Since each Custom Menu is bound to whichever specific Window context it was created from, creating it from exactly the right tab/area is what actually scopes it correctly.