How to get completion callback from MFMailComposeViewController when it's presented by a UIActivityViewController?

Question

I think I can use the completion handler of the UIActivityViewController (haven't tested yet), but that would only give me a BOOL for whether the activity completed or not. I want to be able to handle the error if there is one, as I would in -[MFMailComposeViewControllerDelegate mailComposeController:didFinishWithResult:error:]. Is there a way to be notified, without having to mess with private APIs? Looks like UIMailActivity is the delegate for the mail composer that gets presented. Any official way to do this?