I occasionally want this, too. I think giving the full-Conpherence interface a secondary "Preview -> Post" workflow might be reasonable, so you can either post directly or review a modal preview on your way.

+--------------------------+
| chatty chat chat         |
+--------------------------+
   (Preview) (Send Message)

That would replace the text box with a preview:

+--------------------------+
| preview of chat chat     |
+--------------------------+
      (Edit) (Send Message)

Trying to cram this into the chat column feels too messy and not useful enough, and doing live previews feels hard (where do we put them?) and not necessary (at least personally, many messages I send I don't care too much about previewing, it's just more composed email-like messages that I'd like a preview of).

I also don't think editing Conpherence messages is necessarily completely off the table, but it's probably somewhat involved to make work in realtime and might merit a stronger UI treatment (make it very clear that the message was edited) if we do eventually pursue it. I'd want to push it out a couple of iterations at a minimum.

I wonder if the full-screen editor should perhaps be split 50/50. So in tighter places you could just go full screen.

Basically, it seems beefing up the fullscreen editor makes the most sense since it solves the problem everywhere we may have it (like Pholio comments).