Important to remember where the jurisdiction of the aircraft lies.
I remember a ramp agent in Frankfurt informing the smoking members of the crew that smoking was forbidden on the ramp. They were in the cabin but the smoke was being blown out of the door and that was his point.
The captain pointed out that what goes on onboard the aircraft was his responsibilty and that he was welcome to wait outside (in the rain).
It is important to check the insurance coverage.
Smoking during turnaround is permitted by the company, subject to obvious conditions, re-fuelling or obstructing cleaners etc.