I found, or rediscovered, a page in the back of the Dragon Warrior II "Unveiled Secrets" guide that Enix made way back in the day. On the very bottom, it said I could mail in "For Information About Dragon Warrior III" and they'd send a packet back.
Did anyone ever request this packet? I did get a packet of information on Dragon Warrior II, and that one came with a letter and a map, so I wonder if the one for III had something similar.