How you could have heard of him: Grant Morrison used him as a member of the League of Assassins in Batman Inc. vol.2 #4, but only eagle-eyed trivia kings are likely to have spotted him.
Example story: Detective Comics #408 (1971) by Len Wein, Marv Wolfman (and his annoying use of 2nd-person narration) and Neal Adams
Batman writers have often used hallucinogenic villains to screw with Batman's head, and that makes sense because his world is entirely conducive to psychological exploration. So why hasn't Dr. Tzin-Tzin been used more? Is it because the Scarecrow can fill that niche more effectively? Surely, if that were true, no one would have felt the need to create the equally forgotten Mirage or Spellbinder. Is it the inherent racism of an Asian character, even an Asian-American like Tzin-Tzin, taking on a stereotyped Fu Manchu persona better suited to the Golden Age and its tales of Yellow Peril? Whatever the case may be, even with League of Assassins support, Tzin-Tzin has failed to get much play.
Who else? At the time, Dr. Thirteen and Dolphin were probably as obscure to me, but they've made good since. The next character I do, also from volume VII, will be someone who I think only reappeared in Crisis after his last appearance decades before.