Maybe it would be simpler to *not* bundle lilypond with denemo on mac. Most people are going to place their Lilypond.app in /Applications or ~/Applications or somethings. If lilypond is placed there, I would be able to predict where the executable is. If lilypond is not found, we could have the print view tell us that lilypond is not installed. This could open up a pdf with a link. It will open in the browser and download lilypond when clicked. If lilypond is located somewhere else then we could have a browse button find its location.
Jeremiah