I personally like the idea of a minister of Ao who worships another balance themed deity, perhaps a natural one. It wouldn't be actual worship of Ao for Divine powers but rather as an all encompassing entity or state of being of all things, a balance. Very old greek idea that.
You could describe it as a heresy within the church of said deity, with the nuance that is apparantly a permitted heresy because the deity continues to supply divine power. I wouldn't find that all that far-fetched because Ao himself embodies what a balance themed deity would aspire to be: A keeper of balance.
Perhaps that works?
Or instead of a natural themed deity you could look at the balance domain deities: Auppenser, Oghma and Waukeen.
http://forgottenreal..._domain_deities
They may also support such heresy within their Church. Although I imagine they'd want it nuanced to ensure they appear in the picture as well. Otherwise they'd lose the worship. (e.g. worship of Ao through Auppenser, Ao's aspect on Faerun... or somesuch)