Is that so? I was under the impression that "eau" is used instead of "au" when it's at the end of the word (beau, seau, oiseau, chapeau...)
In all these words, the e has no effect on pronounciation and is there for reasons of historical linguistics, deriving from earlier bel, oisel etc. Gau vs Geau, on the other hand, does effect pronounciation.
Because for some reason - no one is entirely sure why - Cajun surnames in -u (typically in -eau) were all pluralized at some point 200 years ago. "Geaux Tigers" etc is basically saying "Go Tigers (oh and btw we're from Louisiana and proud of it)".