The good news from this end is that your collective wisdom and a text message to a colleague who knows ALL this stuff (but who is working offshore in Qatar and is sometimes unreachable) set me on the correct knowledge path. I was able to find (the only) six 15 litre Nitrox-compatible tanks with DIN valves available on the island, so my customers were able to use those for their dives--we filled them at night. Our DIN valves are indeed the "standard" ones since there is no regulation stipulating valve size here in Thailand. My customers (an instructor + tech/wreck diver and his new bride) had a fantastic time diving with us here and are happy. Since the whole objective of my question here was to provide satisfaction to my customers, we are all winners in this thread.
Many, many thanks to all of you.
Many, many thanks to all of you.