It appears that one could use UIAccessibilityPostNotification for TTS thus:
UIAccessibilityPostNotification(UIAccessibilityAnnouncementNotification, @"the sky is now blue");
However, this raises two questions:
1) Is this allowed by app store rules? Seems mighty close to using a private API...
2) My uses want TSS, not VoiceOver in general. Is there a way to temporarily enable VO, do the TTS call, then disable it?