The only parking brake that could be feasably used as an emergency brake I have ever seen was a transmission (output shaft) mounted hydraulic disc parking brake on a 40s hotrod. One of the trickest looking things I've ever seen. The owner told me he could bring the rig to a smooth stop using nothing but the parking brake. It looked way cool spinning down there under the car while it was moving. Not a very good thing for rough roads I would imagine though.