Lets see...
Coulda sworn celeste byrd did one...
Mirror's Multiplied by Jared Ornstead
http://www.fanfiction.net/u/40569/
(author has a very large anti-akane bias)
Lesbian 1/2 (warning lime) by Donny Cheung
http://www.geocities.com/Tokyo/Ginza/4537/
Life and Times also fits i think (except i think in that one he ends up married to basically the entire sm cast too...)
There is one i believe "Bet" entry where everyone ends up married to Ranma... either blue side or somesuch then again the bet had many x chars end up with ranma it wasn't funny...
Really, is polygamy too hard to see as a cannon-plausible event?
as for being canon? Nodoka would definitely see multiple mistrresses as manly but then that could be the fanon must get grandkids that floats around
but otherwise probably not
(really comes down to what rules Amazons follow and really whether at the core any of the fiancee's are willing to share)
Through the sands of time I walk. Beyond time and space i call to those who awaken to the darkness and realize they are the light.