Hi Daniel !
I see anywhere this question in the dorico forum so :
When Yamaha Volcaloid Plug-in ( Like Vocaloid for Cubase ) Will be integrated to Dorico ( Vocaloid for Dorico ) ?
Just an integration like Halion Sonic Orchestra inside … Yamaha can help you 
That will be very usefull for play back Word Song in the score !
Later,
VST Language ?
Or better
The real tool will be a usual language writing based on XML and the International phonetic language for all language.
( synthetique phonem, phonem voice like airport messages, vocaloid technology or not, ARA formant support … )
With a standard plateform define by the midi.org accross all the sound and vst editor … like musicXML
enhancement for Midi 2.0 or 2.1, VST3 ?
NB : Some editors open the box but not for a standard of communication with midi or musicXML or a specifique XML scheme.
Only a dream ?
Your vision is interresting
Utau maguro