This is a technique that may aid in typesetting verses with very similar melodies stacked on top of each other. Dashed slurs can be used to indicate that two notes are sung separately in one verse and slurred together in another.
Notice that the slurred notes are declared twice: once in the main voice and again in the second voice, where they are hidden. It is these two hidden notes that are connected with a dashed slur, leaving the notes in the main voice free to align with any lyrics. The thickness of the slur is also increased, to aid visibility.
To align the lyrics to the correct notes, place \skip instructions where lyrics should not be placed. The snippet demonstrates that in the \lyricsto mode durations are ignored, so the argument to the \skip command is arbitrary; it will always skip the next note opportunity.
Ideally, this solution can be developed into a music function of its own that takes the notes to be slurred as an argument.