Best way to reference ConfigurationManager.AppSettings in XAML

Is there a built-in, easy way to reference configuration settings in a XAML
file? I'm already using resources and constants a lot, but I'd like to also
move over some other markup to be sourced out of the app.config file. So
far, I haven't seen any kind of extension or data provider keyed to the
ConfigurationManager; closest I've gotten is using ObjectDataProvider, but I
cannot pass in my AppSettings key to it (the indexing is a problem, I think)