@PointC, @OSUblake, @Carl, @lennco
Having spent sometime exploring your various suggestions I have realised I was approaching the problem from the wrong angle. The answer has been to not try to integrate Greensock tweening WITH / INTO Babylon.js renderloops but to use Greensock tweening for all the diving animation and using timeScale to stop and start the dive as needed.
I feared a Greensock tween would not allow other Babylon.js operations while it was being executed but this proved not to be the case.
So with your help, the problem is solved.
Again many thanks.