try 161sex.com and enter 'Anal' in the checkbox....most of them are older and not so attractive :-) which is a shame since I enjoy the 'backdoor' also.
best choices here (on sex141) are probably:
Irene @ National in Jordan (around 700 for the backdoor) - not great to look at, but very acceptable.
http://www.sex141.com/en/viewid. ... ype=-1&keyword=
Michelle @ Hop Yee in Causeway Bay (500+)
http://www.sex141.com/en/viewid. ... ype=-1&keyword=
LokYee @ Observatory in Tsim Sha Tsui (~600+) and depends on her mood!!
http://www.sex141.com/en/viewid. ... ype=-1&keyword=
You can also try the ladies in Sham Shui Po/Mongkok and some of the hotel girls but for all of them its touch-and-go as to whether they will or won't.
There used to be some other ones around, but they have either left the business or are no longer advertising it unless youre a regular.
Happy Chocolate-starfish hunting!!!