In each Office Settings... having a add-ins section where all installed add-ins with a declared OptionPane would appear.
Selecting the OptionPane would display it aside the list of add-ins with the one being displayed highlighted.
In the OptionsPane, the add-ins would be able to have an interface to configure its parameters (For example, for a Zip based add-ins, the user would be able to set the compression method or the default compression level).
Another example:for a filtering add-ins, you could select which mailbox or which folder it applies to.
The declaration in manifest would be similar to TaskPane, but it would be displayed in a different context.
TaskPane is a bad place for that and buttons is not a better place either. Settings should be in an unique place: Settings...