In the long list of interface features that seem dumb until you try them out a while, there is the "Message History" button in Trillian. You hit the button when you're in a chat window, and it opens that channel's chat log in a text editor. After a while, I'd been wishing for something like that in mirc, so I bit the bullet and implemented it using the Popup menus.
In the Popup editor, put this in the Channel tab:
...and this in the Query tab:
I had never fiddled with the mirc scripting language before, and I did not greatly savor it once I did; but I find that it's sometimes a good exercise to try to do learn just enough of this or that little language to do something useful in it and then get out.