I did the same thing, and also thought Murnane may have been the correct name!I looked last night for any footballer in Australia named Mornane between 1900 to 1930 and I cant find any credible reference to a player Mornane playing. I thought the player might have been called Murnane rather than Mornane and the name Murnane was far more commonplace.
Mornane only played one game for Fitzroy. If he was the player identified as "Caldwell", it's odd that after a debut said to have been promising, that he disappeared.