I did ask one of the Ineos guys at the recent Hanging Rock drive event in Melbourne, and he confirmed that you can lock and unlock the centre differential without having to put the gearbox into neutral.
Having experience with constant 4WD Land Rovers for more than 20 years, including rebuilding transfer cases, if the setup is similar, then you shouldn't need to even get off the power to lock or unlock the centre differential (provided you do not have any wheels spinning). With the Land Rovers, when you push the lever across to lock or unlock the centre differential, all that you are doing is applying pressure to a spring that pushes against the selector fork for the differential lock. The differential is not locked until the teeth on the locking ring line up with the teeth on the front output shaft, which is when the spring will push the locking ring across locking the front output shaft to the centre differential. If you are driving on a straight road with no wheel speed differences, it will not lock until you have some difference in axle speed, like turning a corner, which could be long after you have pushed the lever.
This video shows the front output housing of a Land Rover LT230 transfer case and what happens inside when locking and unlocking the centre differential:
As for when to lock the centre differential, I have always locked it as soon as I have left the bitumen and then unlocked it when I'm just about to return to it. As others have said, the centre differential is not designed to differentiate axle speeds when wheels are spinning.