I think it would be better to do this as something that doesn't require special syntax at first, ie. some identifier.
We can always add special syntax later, if this becomes a very popular feature. Also, it looks like the transition-engraver is almost a literal copy of the hyphen engraver. Wouldn't this be much simpler if you'd implement the transition as a layout tweak to a hyphen? You'd get something like: vowelTransition = \once \override LyricVoice.LyricHyphen.style = #'transition a \vowelTranstion -- b https://codereview.appspot.com/565750043/
