I was provided a plugin that does what you want. It's actually really nice and easy to set up.
Download it from here: http://qon.pennmush.org/chatgrabber/
Once you enable the plugin, you'll have to set it up.
Right click the mini windows and hit 'update channels'.
After that's done, right click again and hit channels and choose the ones you want to send to the window (likely all of them, you can shift+click or ctrl+click)
That should default send them to the mini window.
If it's still echoing in the main window, click echo to turn it off.
You can send pages and local OOC to there under the Other communications tab.
I've been testing it this morning. Seems like it also linkifies URLs.
To resize, the bottom-right of the window is a sizing block. Click it and drag to resize.
And enjoy. Seems like it grabs all colors and everything too.
EDIT: You may want to change the font as well; it starts out really small. I just set it to FixedSys at 9 like the core MUSHclient and off we went.