Android OS Accessibility allows developers to integrate their own tts engines into eyes-free api. I want to add a custom text-to-speech engine (and voices) also into ios environment, with the same API used by Voiceover. Lets say, I just want to go to
Settings -> General -> Accessibility -> Speak Selection -> select custom X engine & voice
Is it possible?
No, it is currently not possible. If you are interested in the answer to this question changing in the future, I suggest you file a bug with Apple asking them to implement a publish such an extension point (it would take effect probably not only in VoiceOver, but also in Siri and in general in AVSpeechSynthesizer). I am sure some TTS companies already did file such a bug (at least), but the more people complain about it, the more likely or soon this feature will be implemented.