@JayHawkFanToo I think it seems fairly clear that the rule change for free throws was instigated by Tex Winter because he thought Chamberlain could do it. Below is a link to a Bill Mayer column from March 28, 1956 which gives some details.
As to whether Chamberlain ever did do it, Dick Harp said no in the Mayer column. I found another article from the Des Moines Tribune also from March 28, 1956 which said, "Harp laughed when asked if Wilt could dunk a free throw. “I’ve heard that story,” he said, “but I’ve never seen him do it, either in practice or a game. I think that’s one rumor you can lay to rest.”