Prior to that you have Dream of the Endless showing up in JLA, and Constantine appearing in the background of things like Underworld Unleashed and Hal Jordan's funeral.
Now they can reference things but not directly have them appearing.
So they can reference the Endless, Swamp Thing and the events of the Lucifer comic at the beginning of the Day of Vengence tie-in, but they can't actually show up.
Though Kid Devil was shown to be reading a Hellblazer comic in Teen Titans at some point.