A number of DCU characters are tied in with the Great Disaster: the Atomic Knights, Hercules, OMAC, Kamandi, the New Gods, the Olympian Gods, perhaps Hex, and (indirectly) the LSH, Green Lantern Corps, Tommy Tomorrow, and a couple of Sandmen.
The Atomic War that was part of the Great Disaster took place in October 1986 (see "Hercules Unbound" or the Atomic Knights series in "Strange Adventures"). The Atomic Knights appeared with Hercules ("Hercules Unbound" 10), who was freed during the early days of the war. In that issue, Hercules and the Knights find the water stolen by Skuba in "OMAC" 7-8. But OMAC's world was in the technological near-future ruled by corporations, and Hercules and the Knights are post-war; the Hercules issue takes place right after the OMAC issues. Finally, the Atomic Knights and Hercules were retconned out of existence in 1983 in "DC Comics Presents" 57, where it's revealed that the whole future of the Knights was just a dream of Gardner Grayle. So that would imply that OMAC and Kamandi would be gone too, but both appeared with Superman later (OMAC in DCCP 61 and Kamandi in DCCP 64). Who knows. It was all erased in the Crisis, anyway...
...which took place in 1985-86. Hmmmm...
Post-Crisis, OMAC's grandson Kamandi gets found by Capt. Horatio Tomorrow, and Kamandi gets renamed Tommy, rather than going out into the world that's filled with mutated sentient animals.
Anyway, Hex appeared to take place in the same post-atomic war future that the Atomic Knights did, but I don't recall if there was any direct or indirect connection. And of course, the Legion met Hex in issue 10 while trying to get back home from the 20th century.
The Great Disaster, by the way, was started when Darkseid fought Zeus for the Anti-Life Equation. The presence of Ares causes World War III.
Then there was "Superman" 295 (Jan. 1976). Story entitled "Costume, Costume, Who's Got the Costume?" by Maggin/Swan/Oksner.
Basically, a guy dressed as Father Time steals Superman's costume and vanishes. Superman goes to the Fortress (in a non-indestructible costume) and uses the Time-Screen to find it (by focusing on the JLA signal)... 1000 years in the future, in the ruins of Metropolis.
So he breaks the time barrier and "through the worst of all possible worlds he tumbles... an age undreamed of in the wake of worldwide calamity [picture of humanoid animals]... a time a century hence when humankind is nearly left extinct and mutated intelligent animals fight each other for domination of the ruined planet.. till a race of super-developed men and women arises nine centuries later to overthrow their animal overlords... It is here, in the ruins of "ancient" Metropolis, that Superman touches ground..."
So he gets to 2975, but it's not the Legion's time. Jaxon is wearing Superman's costume, on the steps of the ruins of the Galaxy Building. They fight, and Superman remarks that "You're no ordinary Earthman! Your ancestors must have come from another planet like I did!" Jaxon replies "The Words of Knowledge fall from your mouth as if you were a priest.. but only a mad priest would fight the wearer of the Mighty One's clothes!"
Jaxon claimed to have liberated the costume from the man-gorillas, last seen holding it in "Kamandi" #29. In that story, the Mighty One was said to have perished during the Great Disaster, leaving only his costume. During the fight, 20th Century Metropolis keeps materializing, and when Superman says "Moons of Krypton!", Jaxon drops to his knees, since only the Mighty One knows that name (apart from the priests). Suddenly the costume appears on Superman, the future people disappear, and Father Time shows up.
Only it's really Xenofobe, the Green Lantern of sector 2814 in the 30th century. "The idea was to set right the course of Earth's future... which was upset during a battle the Green Lantern Corps of the future had -- er -- will have -- with a nasty character known as the Time Trapper! The Trapper arranged things so that the energy from our power rings entered the time-dimension, wiping out all earth's possible futures but one -- a future in which a natural disaster ended mankind's reign on earth.. so that the Trapper's foes -- the Legion of Super-Heroes -- never existed! To set things right, I arranged a battle between you and the mightiest of future beings, Jaxon, causing you both to generate power that countered the ring energy, thereby restoring all possible futures! Thus the Legion exists in one possible future, Jaxon in another..."
Or something like that.
In the unpublished Kamandi #61 (which finally appeared in Cancelled Comic Cavalcade), it was explicitly stated that the Legion is in an alternate future of Kamandi. Additionally, Kamandi is an alternate reality's version of Jed Walker, one of Kirby's Sandman's supporting cast (and brother of Rose Walker, of Gaiman's Sandman's supporting cast). I'm not quite sure how that works out, since Kamandi is 100 years in the future, but who am I to say...
Since that was written, a lot of DC history has come and gone (and come back again). But what's relevant recently is the Atomic Knights, the Battle for Bludhaven in the aftermath of Infinite Crisis, and the Command-D bunker buried somewhere below the city.
Diamondrock over at Title Undetermined thought of pretty much the same history connections there as I did ten years ago. Check out his thoughts too.