I'd say it would depend on the campus. If you're at a campus where rush is traditionally cutthroat and spots are limited, I'd say good luck. I know that most houses look for members who will be able to be positive and contributing members for as long as possible -- and generally that's why it's often difficult for upperclassmen to receive bids.

If it's on a different campus where rush isn't as hardcore, then it's another story.

Someone correct me if I'm wrong, but AFAIK, no NPC sorority extends "honorary" membership. You're either a collegian or alumna, but I do know that some sororities extend "inactive" (or whatever you want to call it) status for members who go on temporary leave but have full intention of returning to the chapter.