Data point (I don't think this was explicitly covered above):
Arrived today in UA BusinessFirst (upgraded with GPU). No problem accessing the ZRH Arrivals Lounge. I suppose my *G status might have had an effect, but I got the impression it was the J boarding pass.
Adding another data point: it was definitely the J boarding pass, as I was granted access this week to the ZRH arrival lounge on a BF United flight with no elite status with Star. And it was not a sleepy dragon as she actually scanned the BP on her computer to confirm access.