I can imagine something similar to what you're proposing, however, it works somewhat more like the Windows XP auto grouping taskbar.
Whenever the taskbar gets full, and there are common programs (say Internet Explorer, or word, etc), it places them all into one grouped item.
A similar sort of thing in mIRC could be viable. However, instead of it simply happening when you've got X number of windows open, it could happen based on a similar sort of system as suggested by the second replier to this message.
I quite like this suggestion
