Fitting lyrics to multiple rhythms with ossia stave

Question

I'm trying to transcribe a song with a slightly different different number of syllables in different verses. To indicate this I'm using an ossia stave since writing the verses out multiple times would make the whole thing pretty long. However, the lyrics of the verse with changed rhythm are fitted to the rhythm of the original melody. Is there a way to associate lyrics with an ossia stave?

\version "2.20.0"

melody = \relative c' {
    << { a4 e' e a, | \noBreak b4 e8( d) d2 | \noBreak c4 e e c | \noBreak d4 fis g fis | } 
        \new Staff \with {
            \override VerticalAxisGroup.default-staff-staff-spacing = 
                #'((basic-distance . 4.5)
                   (padding . -10))
            \remove "Time_signature_engraver"
            alignAboveContext = #"3"
            \magnifyStaff #2/3
            \hide Clef
        }
        { \stopStaff s4 s s s | \startStaff c4 e e8( d) d4 | \stopStaff s4 s s s | \startStaff d4 fis fis2 }
    >>
}

verse_one = \lyricmode {
    A -- A A A A -- A A,
    A -- A A A A A -- A -- A,
}

verse_two = \lyricmode {
    B -- B -- B B -- B -- B BB,
    B -- B -- B B B -- B -- B ,
}

\score {
\new Staff = "3"
    << \new Voice = "Lead" {
        \melody
    }
    \new Lyrics \lyricsto "Lead" {
        <<
        { \set stanza = "1." \verse_one }
        \new Lyrics { \set stanza = "2." \set associatedVoice = "Lead" \verse_two }
        >>
    }
    >>
}

Answer 1

There isn't a way to do it, I'm afraid (or at least a practical way).

However, your problem can be solved by a trick that is very handy in complex situations like these. Just add a NullVoice into your staff, so you will have something like

\new Staff
<<
        \new Voice = "lead" { \melody }
        \new NullVoice = "lyricguide" { \lyricguide }
        \new Lyrics \lyricsto "lyricguide" {
                A a a,
                B b b.
        }
>>

The NullVoice is just a dummy voice that you can put anything into but nothing is ever rendered. So you can come up with any rhythm you want, put that into the NullVoice (the notes themselves do not matter, so you can write just a4 4 4 4 | 4 4 4 4 etc.) and align the lyrics to that voice, as shown in the bit of code above.

In this way, you can align the lyrics to any rhythm you want (it doesn't even need to have anything in common with the rest of the voices). You can see this mostly in choral settings where all voices sing sort of the same lyrics most of the time, with some exceptions, but those "common lyrics" actually don't align to any of the voices.