I emailed Greg at Ride Magazine - they did a review of the Zero.7 in the current issue. He emailed me a photo of the BB on the test bike (crank removed) that showed a large hole at the rear of the BB cylinder that would allow water to be drained from the chain stays to the BB, once the BB cups were removed.
But I could see from looking inside the BB on my frame that my frame has no such (internal) hole. I think the difference is because the Ride frame was Di2 specific, whereas mine is mechanical.
So ended up drilling out one of the rivets that holds the cable stop for the RD to the bottom of the DS chain stay.
And.... success! Drained a fair puddle out of the chain stays (they appear to be linked behind the BB so water can flow internally from the DS chainstay to the NDS chainstay and vice versa).
I'll replace that rivet with a self tapper screw.
The water must've gotten in via the holes in the BB for the FD cable. Water probably ran down the cable into the frame.
But I'm just happy I got the river out and can drain it again if it happens again.
I think I know where that water came from (if the frame was from Christchurch). Some of the West Coast's finest, collected over 220k in the wet...