As far as I know hypenation is controlled by the hyphenation dictionary. There are some CSS working drafts on hyphenation, but they give only a few possibilities to link to the dictionary such as
Code:
{ hyphenate-dictionary: <uri>; }
See for more examples
http://www.w3.org/TR/2007/WD-css3-gc...5/#hyphenation
An alternative is adding a soft hyphen after every em- and en-dash, but this is generally not recommended, because many readers make a complete mess of ePubs with soft hyphens. See e.g. the top of this thread:
https://www.mobileread.com/forums/showthread.php?t=28139