Yep, replaced that 'special' o-ring twice, just be sure. New rubber filler neck and gasket, fuel line, all new hoses, some braided, although they only have 2000km on them, or 6 years. Good question about the weeping, I don't know. I was thinking about wrapping everything (hoses, tank, sender, etc) with thin paper to try and identify any minor leaks when driving. There is no recovery tank, DCOEs, and I vent to the atmosphere from the filler neck via the boot floor.
I figured if there were any leaks in the seam I could press on the tank and make it breathe with the vent blocked, but the tank stays nice and firm as if it's air tight.
The search continues.