I hope you've read Ben Piazza's autobiographical novel, The Exact and Very Strange Truth. It's a gem, and so accomplished it's hard to believe it's the only novel he ever wrote. He did write the off-Broadway two-hander Lime Green/Khaki Blue in the late 60's, but I have never been able to find a copy of it.
He gave good performances on screen right up until the end of his life.
He had hoped to open Who's Afraid of Virginia Woolf? on Broadway, but was contracted to the unsuccessful The Fun Couple. Once that closed and Grizzard left to play Hamlet, things fell into place for him.