Passing \tweak-s to engravers

[Urs Liska]

I have been banging my head against the wall for quite some time now, 
but finally I'm at least at a point where I can ask a question here.

I am programmatically attaching a custom grob property to some music and 
want to later act upon that in an engraver.

When I tried to switch the attachment process from \once \override to 
\tweak the engraver suddenly didn't recognize the property anymore. 
First I thought I got it wrong with where the property was attached in 
the music expression, but now I've realized the following behaviour:

When an engraver is consisted to a Voice or Staff or similar context 
only properties created through overrides are visible to the 
acknowledger while tweaks seem to be hidden. However, if I consist the 
engraver to Score also tweaks are recognized.

The following MWE shows this behaviour with a common grob property, but 
it's the same with my custom grob property. For my use case in scholarLY 
it's neither an option to switch back to \once \override (because also 
\tweak-able post-events have to be supported) nor to consist the 
engraver to Score (because I need to know from which context the 
annotations come).

So what do I have to do to make a Voice/Staff-consisted engraver aware 
of \tweak-s and not only of \override-s?
And why is that behaviour as it is?

%%%
\version "2.19.80"

testEngr =
#(lambda (context)
   (make-engraver
    (acknowledgers
     ((note-head-interface engraver grob source-engraver)
      (ly:message "Notehead color: ~a" (ly:grob-property grob 'color))
      ))))

\layout {
  \context {
% uncomment *one* of the following two lines
    \Staff
    %\Score
    \consists #testEngr
  }
}

\relative {
  \once \override NoteHead.color = #red c'
  \tweak color #red c'
}
%%%


Thanks
Urs