GrandStaff does not have "Vertical_align_engraver"

[Xavier Scheuer]

Hi Joe, dear developers,

On the LilyPond French Users mailing list a user asked how to increase
the space between a  GrandStaff  and the following staff (within a
system).
I suggested to use

  \new GrandStaff \with {
    \override StaffGrouper #'staffgroup-staff-spacing #'basic-distance = #20
  }

but it was reported that  StaffGrouper  properties have no effect on
 GrandStaff  (but it works with  StaffGroup ,  ChoirStaff  and
 PianoStaff ).

The only difference I see between  GrandStaff  and the others is that
 GrandStaff  does not have  "Vertical_align_engraver" .

Is this intentional?  Why is it so?
Could we add  \consists "Vertical_align_engraver"  to  GrandStaff  in
"ly/engraver-init.ly"?

Cheers,
Xavier

-- 
Xavier Scheuer <address@hidden>