Metronome Marks above a StaffGroup

[Andrew C. Smith]

Hey everyone,

On larger orchestral scores a metronome mark will appear at the top of each 
staff group (like strings, winds, brass, etc.) Is there a way to achieve this 
in Lilypond? 

I've tried removing Staff_collecting_engraver and Metronome_mark_engraver from 
the score and adding them to StaffGroup, but that didn't do it. I can find 
anything relevant in the LSR or the docs.

Any ideas?

-Andrew