I looked at the InputVerifier class and all I see are booleans. I looked around a bit more (FocusEvent, requestFocus(), focusLost) and I still don't see the particular event that prevents the other Component from gaining focus. I've also looked at all the elements of this and I still don't see it!
I know of a good way to verify the component that would gain focus with FocusEvent.getOppositeComponent(), I just don't know how to manually prevent the other component from gaining focus.
No flags please.