Bill,
I know all states are different, but typically an emergency brake has to be mechanical. It can not be electric or hydraulic or a combination of both. It's this way in Michigan anyway. The line lock is a neat idea for this, but probably not legal?
On a different note, I love your project it's been fun watching it progress!
Your doing a fantastic job. Both you and Ron will be hard acts to follow.
Keep up the great work.