Somebody asked about this a few years ago but the thread was closed with no response (https://social.msdn.microsoft.com/Forums/windows/en-US/9cf1601b-bbca-4efb-8777-525c31e56152/porting-sapi5-tts-engine-to-windows-10-runtime?forum=SpeechService).
Would it be possible to provide some information on how to support 3rd party voices and TTS engines through the Windows 10 Runtime/Mobile Speech API? Support is already available for SAPI5 and there seem to be workarounds to enable OneCore voices as SAPI5 voices but not the other way round (e.g. here https://www.webbie.org.uk/blog/microsoft-speech/). Also, official support rather than hackery is what I am asking about.