Actually, I think I’m on the Road To Success here. I was just reading the .appex docs, and came across this bit:
"To enable data sharing, use Xcode or the Developer portal to enable app groups for the containing app and its contained app extensions. Next, register the app group in the portal and specify the app group to use in the containing app. To learn about working with app groups, see Adding an App to an App Group.
After you enable app groups, an app extension and its containing app can both use the NSUserDefaults API to share access to user preferences. To enable this sharing, use the initWithSuiteName: method to instantiate a new NSUserDefaults object, passing in the identifier of the shared group."
So, heading in to Roll-Your-Own territory, but this doesn’t look terribly complex.