I should clarify. The Turning-Into-Trees picture that is the League's version of falling pearls or that stupid TKJ flashback has Diana in it (or for a while, Dinah). It does not have Supes or Bats. Clark's not in B&tB 29, and while both Clark and Bruce get lines in B&tB 30, they're not actually part of the story. And, as you say, the boys are never on the covers (or, particularly, in the first-page teasers, that I can recall). They were seldom in any of the other, non-tree-related flashbacks either, for a long time. That makes far more of a difference to what we're talking about here - association, thematic resonance, "what a League looks like" - than the theoretical roster. After all, the way you know "what a League looks like" is what you see on the pages and flashbacks and covers, the memorable splash pages and oft-referenced panels, and if the boys aren't getting page time or big battles or cover space, they're not going to be part of that image.