Sorry was on a bit of a vacation, just now catching up on posts.
By writing only to this tag: Suggestions on this site, I believe I am able to restrict the traffic seeing this post to just those on this site.
I don't think this should be done with tags, but I like what you're thinking - a way to cause a post to only appear on a specific site.
This would take coordination with the other websites as well since we all share a common datasource. Each frontend would have to integrate and respect the "visibility" of the post based on what the author chose.
It sounds pretty easy to code, but would likely take some time coordinating and establishing the standards of how it would work. I'll add it to my issue tracker and start some discussions with other devs about it.