Under Tools-->Options-->Display-->Options-->Tray, you can choose to place mIRC in tray when minimized.
I suggest implementing an option allowing the user to pick a second icon for the system tray if he is set to /away.
Then the user could select his own icon (or perhaps a default icon).
AIM and MSN Messenger both show a different icon in the system tray when the user is away. I have difficulty, sometimes, remembering to turn off my mIRC /away message when I return to my computer. ;-)