I don't think you will be able to do what you want Alan. I think you would need to open the drawing once and make the reactor, and then open it again, and then it will fire. Maybe you can make a 'command will start' reactor for the very first command, and then in the reactor call back, have it remove it, so that it will only fire once per drawing.
Just thinking out loud.
Edit: I see you found something while I was typing. Hope it works the way you want it to.