Is there any way to have VoiceOver read a label on command?

You can move VoiceOver focus to an element by using the following:

UIAccessibilityPostNotification(UIAccessibilityLayoutChangedNotification, elementToFocusOn) 

VoiceOver will then parse and read the accessibility properties associated with that element.

You can make VoiceOver speak any string any time you want by calling:

UIAccessibilityPostNotification(UIAccessibilityAnnouncementNotification, NSLocalizedString("QR code has been detected", comment: ""))

Swift 4

UIAccessibility.post(notification: .announcement, argument: "Text")

There is no direct way to tell VoiceOver to speak updates of an element that VoiceOver cursor is not on. This (i.e. speaking the same content "manually") is a feasible workaround.