People who want to post can post, and people who don't want to post can tag other people's (current character's) posts. The list and stuff sounds like a gr8 idea, as does the mingle post.
Repetitive posts are a pretty big concern because who wants to tag the same thing over and over? It'd get stale as fuck really quick imho and I know it'd definitely make doing stuff with fourth wall characters much more difficult for me.
It'd be more work this way but it'd probably be the best way to satisfy everyone.