You can sorta already do that as it is. I suppose you COULD modify the mirc.ini file such as:

alias logdir { if (!$1) { echo 4 -s Error: New directory not specified } | writeini mirc.ini dirs logdir $remove($1-,$chr(34)) }

But then again, it would most likely take a restart of mIRC and could have some SERIOUSLY DAMAGING effects from doing so, as shown like so from the mIRC .hlp file:
Warning: Do not use this command to modify any of the INI files currently being used by mIRC.

But a good suggestion nonetheless, but if it is implemented, you might as well make /mp3dir, /wavdir, /oggdir, etc.

-Zelda4ever
aka "The Big 'Z'"