I saw a seat somewhere with a working handbrake, but I can't find it again. The maker had used a cheap PS2 style controller, wired up the handbrake button of the controller to a microswitch, attached the microswitch to one end of a handbrake, attached the handbrake to his cockpit chair on a rotating joint and added a spring for resistance and return pressure.
He then of course had his wheel, which would require no modifications, attached to the USB port as normal.
Basically, he pulls the handbrake up and this closes the microswitch, which has the same effect as pressing the handbrake button on the modified controller would have. He lets go of the handbrake, the switch is open again, the handbrake is no longer applied in the game, and the spring returns the actual handbrake lever to its original rest point.
It may have been on GT Planet, or on another forum, but I can't find it.