There is a pure python hyphennate.py that uses the standard tex based hyphenation dictionary (like Libre/Open Office, Sigil, etc). So yes a hyphenation dictionary would be a very easy to do and nice edit plugin for Sigil.
See this link:
https://pypi.python.org/pypi/hyphenator/0.5.1
That code can be unpacked and hyphenator.py can easily be added to a plugin. It is LGPL so it is license compatible with Sigil.