\version "2.19"

autoextenders =
#(define-music-function (lyrics) (ly:music?)
   (let* ((has-hyphen?
           (lambda (event)
             (let* ((art (ly:music-property event 'articulations))
                    (is-hyphen? (lambda (ev) (eq? (ly:music-property ev 'name) 'HyphenEvent))))
               (find is-hyphen? art))))
          (has-extender? 
           (lambda (event)
             (let* ((art (ly:music-property event 'articulations))
                    (is-hyphen? (lambda (ev) (eq? (ly:music-property ev 'name) 'ExtenderEvent))))
               (find is-hyphen? art))))
          (add-extender! 
           (lambda (event)
             (ly:music-set-property! event 'articulations
               (append (ly:music-property event 'articulations) (list (make-music 'ExtenderEvent))))
             event)))
     (music-map
      (lambda (event)
        (if (and (eq? (ly:music-property event 'name) 'LyricEvent)
                 ; do not add extenders below hyphens
                 (not (has-hyphen? event))
                 ; do not duplicate extenders
                 (not (has-extender? event))
                 ; do not add additional extenders after _ (internally equivalent to " ")
                 ; to avoid killed extenders
                 (not (equal? (ly:music-property event 'text) " "))
                 ; do not add extenders after "" to allow ending extenders
                 ; within _ _ _ _ skip sequences with _ _ "" _ _
                 (not (equal? (ly:music-property event 'text) ""))
                 ; ditto, with _ _ \markup{|null} _ _
                 (not (equal? (ly:music-property event 'text) (markup #:null)))
                 )
            (add-extender! event))
        event)
      lyrics)))

\score {
  <<
    \new Staff <<
      \new Voice = "melody" \relative {
        \voiceOne
        e'4 e8 e c4 c
        | c e8 d c2
      }
      \new Voice = "splitpart" \relative {
        \voiceTwo
        e'4 c c c
        | c c c2
      }
    >>
    \new Lyrics \lyricsto "melody" {
      They shall not o -- ver -- come,
      shall _ not!
    }
    \new Lyrics \lyricsto "splitpart" {
      \set stanza = "no extenders (old default)"
      We will _ _ _
      will _
    }
    \new Lyrics \lyricsto "splitpart" {
      \autoextenders {
        \set stanza = "with extenders"
        We will _ _ _
        will _
      }
    }
    \new Lyrics \lyricsto "splitpart" {
      \autoextenders {
        \set stanza = "with extenders, plus corrections"
        We __ \once \override LyricText.self-alignment-X = #LEFT will __ \markup{\null} _ _
        \once \override LyricText.self-alignment-X = #LEFT will __ "" _
      }
    }
  >>
}