I'm thinking something very simple:

word.word_id = 1
word.word = "t-shirt"

synonym.word_id = 1
synonym.synonym = "tee"

synonym.word_id = 1
synonym.synonym = "tshirt"

so searches on "t-shirt" return products also containing string "tee" or "tee shirt".

I haven't looked at the Zen search code at all yet ... if anyone can point me in the right direction it'd be appreciated.