I think anti-rioter posters need to step up in English in venues such as Twitter and Facebook so people who don't know about the rioters' dirty deeds have information sources in English other than the vast majority of pro-rioter biased English language media.
I don't think it shall make any difference. Who are you trying to convince ? Who are you trying to preach to?
Confirmation bias is a huge thing. People always operate on some set biases,prejudice and understanding and they shall resist mighty to see that they were right all along. Even if Truth makes itself clear, they shall miss it.
Those who are in search of truth OR are really open to see the finer details of an issue will always find it. But such people are rare. Most are simple folks- just like many HKers or many other Humans around this planet. They don't care.
That being said ... Change in popular opinion can indeed be inflicted. Such a change needs either time or impactful events or both.
The Only people who needs to know fully about the precarious situation are the common folks of the mainland. Theirs is the only opinion that matter. The disparity in the reporting, the biases in the media outside of the GFW are all on full display. And burning the national flags and attacking mainlanders ... Oh my oh my... Talk about overkill.