There's actually a W3C
draft:
EPUB 3 Text-to-Speech Enhancements 1.0
IVONA 2 Text-To-Speech has SSML support:
SSML Support in Ivona Text-To-Speech
Some Microsoft SAPI voices also have limited SSML support.
Improve synthesis with Speech Synthesis Markup Language (SSML)
However, AFAIK, there aren't any
epub3 apps with SSML support.