That's actually one of my big concerns, too. Not because Disqus doesn't handle this stuff well, but because it creates a potential point of conflict.
So, basically, you can set up a Disqus account to display no threads on Disqus itself. This, in turn, allows you to do the permissions thing entirely on the client/display side. However, this does lead to potentially weird conflicts where (say) a post is public on LJ and private on DW, or your list of permitted viewers isn't synchronized between the two services and someone who can't see it on one can see it on the other.
Essentially, this would attach the thread to a post, and the thread would only be visible to people who could see that post, but because you're actually posting across multiple services it's possible that people will be able to see the post in one place when they shouldn't unless you're scrupulous about keeping your permissions tightly policed.
So, your worry is pretty much spot-on. Because this system doesn't implement any security on the synchronization, if you're screening in one place and not in the other then the screening will only take place in one of the locations. (Actually, screening in general is probably going to be hard to handle... I'll have to think on that one.)
Additionally, this is a pure back-end code, so there's no real guarantee that anyone will adopt it. And while I may live in hope, I actually think it would be hard to convince LJ to push it into their system. After all, they benefit pretty significantly from the fact that some people won't jump services because it would mean losing easy access to certain conversations.
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So, basically, you can set up a Disqus account to display no threads on Disqus itself. This, in turn, allows you to do the permissions thing entirely on the client/display side. However, this does lead to potentially weird conflicts where (say) a post is public on LJ and private on DW, or your list of permitted viewers isn't synchronized between the two services and someone who can't see it on one can see it on the other.
Essentially, this would attach the thread to a post, and the thread would only be visible to people who could see that post, but because you're actually posting across multiple services it's possible that people will be able to see the post in one place when they shouldn't unless you're scrupulous about keeping your permissions tightly policed.
So, your worry is pretty much spot-on. Because this system doesn't implement any security on the synchronization, if you're screening in one place and not in the other then the screening will only take place in one of the locations. (Actually, screening in general is probably going to be hard to handle... I'll have to think on that one.)
Additionally, this is a pure back-end code, so there's no real guarantee that anyone will adopt it. And while I may live in hope, I actually think it would be hard to convince LJ to push it into their system. After all, they benefit pretty significantly from the fact that some people won't jump services because it would mean losing easy access to certain conversations.