I have not heard any proven cases or videos of the sexual abuse. The only thing were the rumours on LIHKG and Apple Daily, neither which are the least bit credible as sources. Might as well be "Auntie on the MTR".
I believe reading about a hearing in which a protester accused a policeman of grabbing her breasts and there was a video, but it only showed her being pushed in the midst of a scrum or something like that.
Simon Cheng (quoted in the above article btw), this guy is great. There is a reason why his case was not widely publicized any more. He initially claimed he was tortured (strapped to a tiger chair and also crucified). Then the SZ police released the tape of him going to the brothel, which pretty much made him a pariah. Although his confession could be questionable, certainly where he was caught was not. No one called him a "spy" certainly not the SZ authorities, and there was no proof of torture. Contrary to popular belief, you cannot be beaten without leaving bruises. I personally hypothesized that he might have been taking part in some subversive activities, which might explain why he was picked up.
You could make the argument that visiting prostitutes really isn't much of a crime, and I would agree, but in the end that's what he was caught for.