REQUEST: \whiteout using grob stencil instead of bounding box

[Kieren MacMillan]

Hi all,

Is there any [easy] way for the \whiteout command, and/or the #'whiteout 
property, to use a grob's stencil outline instead of its bounding box?

whiteFFMarkup = \markup { \whiteout \pad-markup #0.5 \dynamic ff}
whiteFF = #(make-dynamic-script whiteFFMarkup)

\paper { ragged-right = ##f }

\score {
  { s4-\tweak #'extra-offset #'(2 . 4.5) \whiteFF }
}

I would love to be able to set a #'whiteout.margin (or .border or .thickness or 
whatever) property, and have it simply "outline" the grob. Of course, if this 
were implemented, it would be great to retain the current behaviour (i.e., the 
bounding box whiteout).

Thanks,
Kieren.

p.s. Mike: If you've got time for some more bounty work, this might be a great 
place to start!  =)