Johns's Superboy in the 30th Century idea? Oh, that surfaced during the JLA/JSA/LSH crossover "The Lightning Saga" and was made more explicit during the "Superman and the Legion of Super-Heroes" arc. In the latter, we see a teenage (and lonely) Clark being approached by Cosmic Boy, Lightning Lad and Saturn Girl and invited to go to the 30th Century for a visit. And the rest is history.
It's a good patch, really, which allows the pre-Crisis Legion continuity to be more or less preserved intact (with a little juggling to deal with the few stories that took place in 20th Century Smallville, but that's easily done for the most part).