Apps like Pidgin or Empathy can be reduced to the notification center, evolution or xchat can't, it need to have an another icon in the systray that i don't want.
Write a patch. And actually, Evolution can't go to the notification area (it's not a tray in GNOME) anyway. It doesn't have an option to iconify to start with.
But yeah, the only way to get them to emulate Empathy's broken close behaviour is to write a patch.