From what I have been able to find out regarding the unavailability of Coke Zero, it has mainly to do with the flooding at Thai Pure Drinks Rangsit Klong 13 plant. Essentially this is the only plant that bottles Coke Zero for Thai Pure Drinks. They have another plant capable of bottling the sugar based product which is running flat out and has no spare capacity available to be retasked.
It will likely be at least 1 month after the factory becomes dry before production can begin again. As of today, this factory is still closed and flooded. I can only guess when it will reopen. My house is in a different area of Pathumthani and still under 1.2 meters of water. I would not be surprised if Coke Zero remains essentially unavailable until the end of February.
Thai Pure Drinks has the license to bottle and sell Coca Cola throughout the entire northern and central areas of the country. A different bottler Haad Thip, has responsibility for the southern provinces of the country, and they should still have the capacity to produce Coke Zero. There is no connection between these 2 companies however, so it is not possible to divert product from the south to the northern and central areas. But if someone wanted to make a trip to Had Yai logic suggests it should be possible to purchase Coke Zero in large quantities.
Edit: Haad Thip has the world's most annoying website, but according to their map the northern most province where they are allowed to sell is Chumphon. If anyone is coming back from that area to the Pattaya/Chonburi region and can pick me up several cases I will pay a premium for it.