I doubt we'll see a ring slinging Hal Jordan in the flashbacks, just a cocksure cruising for a bruising, won't his comeuppence be hilarious, test pilot Hal Jordan. Of course Oliver can ram heads against a lot of pre-origin Green Lantern Super villains, or even deal with a cell of Manhunters... If Oliver had an adventure with Abin Sur, he could really look down his nose at Jordan in the present as that noob is still learning the ropes. HA! Professor Stein was Sinestro! Or at least, Victor Garber is both of them. Carol Ferris is an heiress living one city over. If they didn't grow up together, then they probably f... Give this one to Tommy. Yeah, Tommy and Carol would have been great together.
I don't think continuity would work if they introduced the Lanterns in Arrow flashbacks, but they could set up the backstory nicely with a hunt for crashed alien spacecraft, a mysterious lantern shaped artefact and a a pilot named Hal Jordan going missing at the end. Edit: They could even lay the groundwork for Ollie's and Hal's friendship already. Were they friends before becoming superheroes in the comics?
I'd be shocked if we didn't get some kind of Hal Jordan appearance before the end of the shows. I don't know if they'll go the full on emotion powered rings and aliens route, but Hal and Ollie's friendship seems to be just to big a part of the mythos for it to not pop up on the show at some point. We have gotten the Flash using his powers, and the Lazarus Pit, so I wouldn't put the more sci-fi elements of the Green Lantern stuff totally past the show.
Probably. Somehow, she can sill wear size 6 pumps. Women. Sheesh. They're not doing fashion right, until it hurts.