my thinking is;
The basis of Chi Sau probably goes back to Leung Jan and the generation after such as Chan Wah Soon and Leung Bik. My reasoning is that some of the mainland groups did have a primiative form of chi sau.
YKS and IM probably developed the Poon Sau platform and I would bet they got help from other wing chun people of thier generation.
IM kept developing chi sau as you can see the development especailyl with HK guys and WSL. The Foshan students of IM did not have Dan Chi, only Lap Sau, Poon Sau and Chi Sau. Some schools also have the bamboo dummy.
I think the early 1900s was a real developing stage for southern Chinese Martial Arts with places such as the Chin Wu Association springing up and the sharing of knowledge. I would bet that a spark for chi sau came from taiji push hands. Saying that, push hands and chi sau share many theories but are completely different animals.
Regards
Paul