Selective ambitus in staff context

[aftertheflood]

Hi,

is there a way to include some voices into the calculation of the ambitus 
engraver for an entire staff whilst excluding others, be it named or throw away 
voices?

In the example, I would like the ambitus to be displayed as
c' - a’
as if I had used a CueVoice for the instrumental part. Or is it easier to tweak 
the CueVoice so that it is rendered like a regular voice?

%%%  SNIPPET BEGINS  %%%
\version "2.25.25"

vocStaffMusic = \relative {
  c'2 d
  \new Voice = "hummingVoice" {
  e4^"hum melody" f g a
  }
  % \new CueVoice = "instrumental" {
  \new Voice = "instrumental" {
  e'4^"flute" f g,,^"horn" a
  }
  c2 d4 e
}

vocLyrics = \lyricmode {
  Ly -- rics
  more ly -- rics
}

\score { <<
  \new Staff \with { \consists Ambitus_engraver } {
    \new Voice = "vocWithLyrics" { \vocStaffMusic }
  }
  \new Lyrics \lyricsto "vocWithLyrics" { \vocLyrics }
>> }
%%%  SNIPPET ENDS  %%%

Thanks,
Oliver