Scheme Tweak Help: Highlighting a Staff in a StaffGroup

[emw]

I'd like to write a custom Scheme /tweak that will allow me to set the
background color of a staff within a staff group. 

I am printing out a four-part SATB arrangement, and would like to be able to
highlight the parts individually (to simulate what singers often do-- using
a yellow highlighter to highlight their part within the score). 

I did find the following tweak: 

% move to right position
-\tweak #'extra-offset #'(-126 . -5.5)
% place in bottom layer
-\tweak #'layer #-1
^\markup {
 % specify color
 \with-color #(rgb-color 1 1 0)
 % specify size
 \filled-box #'(0 . 33) #'(0 . 6) #0
}

Unfortunately, the method above isn't really independent of the layout, so
if you manage to get the highlight positioned correctly, and then you change
the margins, the highlighting will be off. 

If anyone can help me write a Scheme tweak for this, I'd appreciate it. Even
pointing me to a tweak that does some of the same things would be helpful
(i.e. tweaking output across an entire Staff in a StaffGroup, etc). 

Thanks, 

     - Eugene
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