On December 31st, 1988, a New Years Special titled “大晦日ゲゲゲの鬼太郎 電リク生放送 生誕20周年SP” aired to commemorate the 20th anniversary of the first Gegege no Kitaro anime. It was a live broadcast. Even in Japanese, there’s very little about this special online. But from what I can find, there were re-runs of fan-favorite episodes (from the 1985 series), and staff members from the 1985 anime answering fan questions over the phone. There was also discussion of a possible sequel to the third anime, based on the manga “Kitaro Kunitori Monogatari”.
I don’t think that anyone here will be able to help with this search, but since there’s no English documentation on this, I want to spread the word about this as much as possible.