Might be best to post your current game.script first, as it's probably been changed from the original.
As I see it, the right click section of the game.script is primarily concerned with checking to see what the mouse cursor is currently hovering over, and then applying the relevant code. Or, in the case of selected inventory items, deselecting them.
In its original form, it first checks to see if an inventory item is selected, and if it is, then it deselects it. If an item is not selected, the script then checks to see if the verb bar/coin is active. If it is, it makes in inactive. If not, then it checks to see if the cursor is over a hotspot. If it is, it makes the verb bar/coin active.
If there is no inventory item selected, the verb bar is not active, and the verb bar is not even needed, then the script runs whatever "RightClick" event is contained in the script of whatever the cursor is hovering over.
If it's hovering over empty space then the script does nothing at all.
So, I think what you want is to do something when the cursor is not hovering over anything but empty space.
I think you can just put the menu loading code after the two IF statements in the right click section of the game.script, so then it will check everything else first, and if none of the above are required, it will then run the menu loading code, and load your menu.