Hunt1 was right. Assuming this wasn't added to the lineage in later generations, here is Yiu Choi lineage people doing Luk Sao Chi Sao.

Yiu Choi Chi Sao at 4:44:
http://www.youtube.com/watch?v=xHlGGSOyNk0

Unfortunately, Ng Chung So lineages don't seem to have survived. Footage I find of Chan Wah Shun/Chan Yu Min lineages don't show Chi Sao. What they do show looks VERY different from Yip Man Wing Chun. So there has likely been some "mixing" since Chan Wah Shun's day.

Yiu Choi seems to be the only lineage connected with Ng Chung So other than Yip Man that is still around. They do Luk Sao Chi Sao, but since Yiu Choi was one of the "three heroes of Wing Chun" in Foshan along with Yip man and Yuen Kay San and they hung out together regularly, it makes sense that he would have been in on any Chi Sao developments. But we cannot rule out the possibility that Ng Chung So was the innovator and showed it to all of them! I guess we'll just have to take Sum Nun's word for it until something surfaces that tells us otherwise. At least that's how I see it. Your mileage may vary!