With production for Justice League wrapped and no actor cast in the role, it is likely that Green Lantern won’t be making an appearance in the team-up film.
However, pickup shoots for Justice League have yet to take place, so a Green Lantern cameo could plausibly be filmed then.
The plan could still be for a brief Lantern appearance in Justice League and WB is just holding onto that secret for as long as they can.
I don’t know (if I’m still in the running for Green Lantern Corps).