So... I've been thinking about Drew's hints here and... here are my conclusions:
1. It is there to be found in game in ED.
Whatever we are looking for is not in one of the permit systems/sectors. It is out there somewhere and can be found now. No need for any future updates. It's there NOW.
2. You do not need 'Horizons' to find it as it was finalized long before 'Horizons'.
See item one. This confirms that we are looking for something that is currently accessible inside the game.
3. Whether it is found or not depends entirely on who goes looking.
Now... this one requires some thought. Drew further said, "It's not locked to a rank. All you need is wanderlust, patience and an ability to read between the lines."
That last bit... "an ability to read between the lines" is, I believe, a most important clue. Perhaps the answer is to be found through a careful reading of the official novels. Kate Russell's "Mostly Harmless" mentions the Prism system and Salome. Perhaps the pages of all (or some) of the books contain clues that, put together, will help us adventurers discover what lies beyond the rift? If this is the case then reason why it "depends entirely on who goes looking" is because the person who finds it will need to have been someone who has read the books and found the clues.