Intriguing idea Hamish.
I've just looked and I don't think so. Even if you linked it to another activity that changed its state based on the external tool activity.
I think you'd have to write a custom course format that uses the activity completion code along with the course renderer to find out if a given module of the external tool type is complete then not show it.
I can't see any markup changes that happen (bar the image icon changing) whereby you could have some custom JavaScript in the theme to remove the appropriate link.