Multiple choice NSPopupButton ?

I've decided (hopefully well received) on using a popup button as a 
multiple selection control. I use the item's tag values as a bit 
position whose mask I keep in index 0; a user selecting this item clears 
the entire mask while other item selections toggle that item 'tag' bit.

It's done like this action:

- (void)processPopup:(id)sender
{
   unsigned long value;
   int i, tag = [[sender selectedItem] tag];

   if (![sender indexOfSelectedItem])
   {
     for (i=1; i<[sender numberOfItems]; i++)
     {
       [[sender itemAtIndex:i] setState:NSOffState];
     }
     [[sender itemAtIndex:0] setTag:(value=0)];
     tag = 0;
   }
   else
   {
     BOOL state = [[sender selectedItem] state];

     // tag at index 0 keeps our mask
     value = [[sender itemAtIndex:0] tag];
     [[sender selectedItem] setState:state];
     if (state)
       value |= 1<<tag;
     else
       value ^= 1<<tag;

     [[sender itemAtIndex:0] setTag:value];
   }

   // Tooltip has our name
   NSLog(@[EMAIL PROTECTED]
"%@[EMAIL PROTECTED]
(%09x) = '%@[EMAIL PROTECTED]
' %d:%d",
     [sender toolTip], value,
     [[sender selectedItem] title], tag,
     [[sender selectedItem] state]);
}


But the question is how can I get the visual to mimic the model? Do I 
need to sub-class the popup button so I can have multiple selections?
-- 
/los "I was a teenage net-random"