Close already exists, although I can see your point about having the logs for each chat room in a different folder.
I get the channel name as part of the filename with the following configuration for the Logging section via mIRC Options
Automatically log: Both
Include Network
Make Folder
Date Filenames: By Day
I don't think it would be hard to create a script that created a directory for each channel, then moving the appropriate files into the directory. (In fact that sounds very familiar, something may have already been done)
