Currently, we have the ability to "bump" a chost, by drafting and then reposting it. This resets the posting time and bumps it to the top of everyone's feeds. Potentially abusable, but if we're assuming good faith, then it's a great feature. You can simply bump a post for visibility without making it appear in two places in the timeline.
Assuming there's no intent to remove this, I suggest leveraging it to make "liveposting" convenient, and in general, to make it easier for people to do twitter (or columbo) style "one more thing" additions to chosts, without taking up a bunch of timeline space with a rechost.
To support livechosting of events in particular, I suggest a little field that can appear under a post - not in the edit interface, but in the "looking at one post that you made" view, with a button that says "Add"
If you put something in this field and hit Add, it appends your new text to the post, separated from the existing content by an <hr> separator, then bumps the post up the timeline.
Value is that this could be implemented without creating a completely new Post Concept, just automating existing behavior - everything I'm describing can be done by hand today, it just takes four steps.
Wiggle points: Should new text append to the top or bottom of the chost? Should new text be prepended with a timestamp? For timeline politeness, would it be best to always append the new content at the top and then automatically follow with a Read More, so the post never grows larger than the latest update?