James Turnbull, Will Buck and I had a session a couple of years ago and figured it out. James managed to get it done, and has a video of it, so worth asking him. I've not been back to it since.
It's one of those that's a lot easier to describe when stood underneath it, but basically you need to rock left a long way, pressing into the groove, then move back right once stood up on the starting handholds. Very weird and not obvious at all. No idea if that's how it was done originally, but was the only method we could make work.