Can't help on the Yang style taiji front, but I know there are a couple of people teaching Yiquan in HK, specifically:
1. C S Tang,
http://cstang.www3.50megs.com/index_en.html Very well known teacher in HK, teaches mainly Gao style bagua but also dragon fist, Yiquan and Taichi. AFAIK his Yiquan mostly comes from Han Xingyuan line, but think he has also visited other teachers.
2. Ng Chi-hang,
https://sites.google.com/site/yiquanintro/yi-quan-ke-cheng He is a disciple of Yip Hei-sing, who was one of the senior students of Han Xingyuan in HK. He has been featured on a couple of HK documentaries about Yiquan. Contact details on his website.
I am also interested in Yiquan and will be in HK next month - if you are planning to visit either of the teachers above maybe we could go together?