Cameos aren't all that necessary, some kind of verbal or visual easter egg type reference is all that's needed. They did it once or twice on Smallville, once where someone was reading a newspaper that had an article entitled " Themiscyran ambassador to address U.N", or on Arrow, where you keep hearing in the background TV news reports about the "Star Labs particle accelerator" being brought online and the protests against it and all that, which I believe is a precursor to the emergence of the Flash later in the season.