I'm thinking at this point it'll be announced in a Playstation Blog post
Would make sense for it to be at PSX more than any other conference but the Star Wars voucher still throws everything off. If those aren't available right when people buy the console people are going to bitch, especially if it's supposed to be a surprise and the card just says "availability TBA"
Unless they just make them say STAR WARS Classics and then when PSX rolls around they say "you may have noticed that a few Star Wars titles blah blah blah, well now we're proud to announce that this was the first taste of PS2 classics on PS4"