When a user types a / as the first character on the input text box I think it is safe to assume that they are trying to enter an IRC command. This assumption can be exploited though the use of the Windows auto-complete functionality (as seen in Internet Explorer's Address Bar); when a user types a / it could display a complete list of all the common commands mIRC is aware of and all of these commands specified in the Aliases files and elsewhere.
As the user continues to type the command (if they don't select one) it will try to complete and if they see the command they are typing they can hit down + enter and the command will be filled in.. note that if the auto-complete box is selected and enter is hit, it does not send the currently typed text but instead appends to the box.
This will not limit what the user can do because new commands will just take longer to type the traditional way.