I agree that a stepper motor of some kind would work well. I like the idea of being able to move it incrementally, so as to minimize vibration. I'm thinking the flash sync would be a good way to detect when the shutter has actually opened; at that point stop pushing and start timing, and then release after 8 sec.
Not really an issue with an eight second exposure.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.