You're talking about 3 requirements.

1. cnc 4th axis
2. Indexer
3. Lathe

From the InTurn web site;

"SPINDLE LOCK:
Custom made articulated pneumatic caliper provided over 100 ft-lbs of holding torque on 90psi shop air.
Cylinder rated for 200PSI leaving a great deal of added capacity, although so far no operations have needed more than 90psi.
DOT approved hardened and ground steel vehicle disk brake rotor is mounted on a thick steel collar and balanced.
Note, the Tail Stock II has a combined spindle damper/lock provided by an internal band brake.
CNC controlled solenoid operated pneumatic valve controls the spindle lock. The valve and a CNC compatible relay are included."

So I'm thinking about a brake disc and caliper from a car scrap yard with an air over oil actuator.