The dialog_options_mouse_click function is a bit misleading IMO as the name would seem to suggest it gets called when you click the mouse on a dialog option. Instead it works if you click outside of any option...
I would actually find it useful to be able to run additional scripts when the user selects a dialog option. Or at the very least be able to check whether a dialog option has been used (read). I was actually trying to implement a custom system for tracking whether the option has been used so that I can make use of the game.read_dialog_option_color in my custom dialog rendering script. Without the ability to capture a dialog option click and/or test whether an option has been used the property is useless to custom dialogs... :-\
That's a good point -- AGS already records this information so it makes sense that there should be a script function to find out if an option has been read.
I was actually thinking that Is might be a bit ambiguous...HasBeen might work better.
Thanks for the response Chris! ;D