Changing the mirc.ini likely won't work anyway. mIRC stores settings in ram while it is open instead of going to disk all the time, so you are correct that mIRC would have to be restarted. But on exit, the changes will probably be overwritten by mIRC flushing settings back to disk. I think that this suggestion is probably useful for some people, though some of that can already be done with the /log command, such as:
/log on #somechan -f c:\path\to\logs\#somechan.log
It doesn't change the log folder for all logs, but with a little creativity and some script, it could be done.
