NOMB, and no offense intended, but I'm curious: if y'all are leery of organized faith, why do you wanted to have your wedding officiated by a "religious personage"?
As for who to talk to, UU's are a good place to start. Some Methodist clergy might be open to the idea, but many would not. You could also check into the pastors of non-demonination congregrations; they do not bear the same hierarchical burdens as clergy affiliated with specific groups.
I would also consider approaching a Native American shaman, especially one who is of a people who traditionally practiced polyamory. Fidelio and I were married by an NA shaman(ess). Beautiful ceremony it was, blending his NA and my Christian cultural heritages.