The titlebar of mIRC is made out of 2 parts: the word mIRC, and your /titlebar text. When your windows (channel, query, status) are maximized, their title is added to your mIRC's title inside  brackets.
The current /titlebar command only lets you change the piece of text that is between 'mIRC' and the brackets, so you can't use it for completely controlling the line.sz.dll
contains the SetTitle function that lets you reset the titles of both mIRC and other windows.
When you are in a #channel, try this command to delete the brackets part -
- //echo -a $dll(sz.dll,SetTitle,$active)
Then, use this command to change mIRC's titlebar:
- //echo -a $dll(sz.dll,SetTitle,mirc > hello!)
If everything worked as it did to me, the only text in your titlebar will now be "hello!".
See the readme file for more information, and make sure the dll is placed in your mIRC's folder before using the commands above. good luck