You may perform the otherwise dirty destructive deed:
With the mask selected:
1) In the Transparency palette dropdown, select Multiply, or anything else but normal;
2) Object>Flatten Transparency.
Oh wow, to my surprise that actually did work very well, although i think I've achieved the same result by doing it a different way without messing with the Transparency option in the past. But this works just as well, Great! Thank you very much for your help much appreciated.
You are welcome, MW.
In connexion with vector artwork the deed may have a downside, namely its outlining of strokes, if any.
If the other way should spring to mind, please post it.