The Message Opened
state is forbidden to change. It is a kind of a system ECM state. You can customize only states of the green area (Productive, Unproductive, etc.).
However there is a way to do what you want. The moment of message open is handled with the help of small image added at the bottom of a mail. Image source is an ASPX page. When a recipient downloads mail images, the ASPX page is requested. You can customize the ASPX page requested by the image (add your code there).