Grigg,
Sort of the same idea...I know a Cat V-80 forklift uses a disc and mechanically actuated caliper, but probably not large enough for your taste.
I would maybe focus on this type of market..industrial lift trucks. There is a company called Horton that makes brakes, but I am not sure they have a mechanical one.
I have seen the photos of your shop and have seen your skill level...you could make the disc quite easily. The caliper in the V-80 Cat resembles a automotive caliper. Two stationary friction "pucks" on one side, and two that are actuated by a cam action on the other side. Pulling the parking brake rotates the cam that squeezes the frictions and centers the caliper.
Don't forget that when you come backwards through the ring and pinion ratio, you gain quite an advantage to hold your heavy truck with less brake.
Stuart
