In many chat clients you can get the last typed message by hitting the up arrow. It doesn't seem to work in this client. Is that not an available feature or is there some way to do it?
Per Colloquy's FAQ:
So; ⌥+↑ or ⌥+↓ gives you the last, or next message, respectively.
And per that Hidden Preferences link, it would appear that you can change it to use up/down arrows only by opening
Restart Colloquy after entering this command if it was already open, and see if that works.