I've tried a few variations on a theme, and I can certainly change lyrics if they already exist, but any attempt to actually add a lyric line causes Musescore to crash.
Is there anything obvious I am doing wrong?
(See attached file...)
Answer, after poking about in the source code: Cursor.add does NOT support lyrics, instead it crashes embarrassingly.
However, Chord.add DOES.
Caveat: Because the Lyrics.no property (ie, Line no) is not exposed to qml, you can only add the first lyric line.