after-last-staff-spacing when "last staff" is a non-staff

[Kieren MacMillan]

[Lilypond 2.13.23]

Hi all,

I've got a classic setup in my choral score(s):

<<
  \new ChoirStaff <<
    \new Staff \fooMusic
      \addlyrics \fooLyrics
    \new Staff \barMusic
      \addlyrics \barLyrics
  >>
  \new PianoStaff <<
    \new Staff \upper
    \new Staff \lower
  >>
>>

It appears that changing the ChoirStaff #'after-last-staff-spacing does not 
affect the spacing after the Lyrics context (i.e., between the ChoirStaff and 
PianoStaff), because Lyrics is not a "staff".
Is this expected behaviour? A bug? Worthy of a "known issue" at least? (I'm 
happy to contribute the doc patch... I just want to know if I'm doing something 
wrong.)

Thanks,
Kieren.