Like imago said, he was shooting in Barcelona so there was no way he could have been at the Con. There wouldn't have been much point in Reynolds showing up at Con anyway. What were they going to say? "Oh hey, we came here to show you some unfinished artwork that might have a chance at possibly being in the film. Just a little treat for you guys." Comic book movies that are 2 years away rarely show something good at SDCC, same thing with Thor and Captain America, nothing new that we didn't already know about the films. This was just like that Scarlett vs. Gwyneth rumor, utter BS.
And on a side note, I picked up Secret Origin the other day and recently finished it, and I would really like to see some of that stuff turn up in GL. It was a very interesting read, and it would be very cool to see some of the images from SO make it into the film, such as Hal landing that jet, and a but-load of Green Lanterns fling in front of Hal as he just arrives on Oa. And I hope that his ring talks like it does in the book. That would be pretty cool, too.