User1 - would your info show what kind of USB interface is used on the camera? It's low probability, but there might be a way to toggle the USB data lines and therefore use that as a signal output from the camera. I can look through the data sheets (if it's a commercial part) to see if that might work.
Doing this would probably make the USB non-functional until the next power cycle which would be an OK drawback, but this would allow the MD script to trigger another camera (a DSLR) for instance.
