Help me understand why the spring "tab" would break in the first place. It keep the spring from rotating inside the hub, but there is no significant rotational force on the spring other than inertia. The keys force the tab in and out of the locating hole when the clutch sleeve is moved during shifts, but they don't rotate the spring. Could the problem be the tab binding in the locating hole?
The keys as well keep the spring inside the hub by design, but without the tab and 6.9K RPM I could imagine how it could be spun out.
As you and many others likely know, the tab/tang/bent part becomes very weak (work hardened) during/after the 90 degree bending operation if the spring isn't formed/annealed/tempered perfectly. That is the real problem here. In many other trans designs, there is no 90 degree bend, there may be a "hump" (Tremec) or a little stubby tang at each end (Borg-Warner), or a full circumference spring (Hondas).
You are right, the stresses going into the tang are not principally from rotation - it is from shifting. Each shift "cycles" the spring, and any vibration is a "micro-cycle" that will tend to reduce the life of the spring. In fact, I have found transmissions with broken springs that have luckily stayed in place enough to prevent a synchro failure. This is the rare exception, though.
If you have the subassembly from my video in your hand, cycle it a few times and watch what happens to the spring. There is friction between the sleeve and the keys, and friction between the keys and the springs. On worn springs you can see the witness marks where the syncro keys and locating tang are being "rubbed". So, the tang is really feeling stress from all directions - but in the "rotational" direction, it is only resisting the compression and relaxation of the spring hoop during the time the keys are cycling from flush to depressed to flush again.
Because we are shifting hard/fast/under higher load, the spring is getting more stress/strain than loafing on the street.
There are other, better, ways to solve this problem, but I wanted a solution that would pass tech in a 1990 Runoffs-winning Showroom Stock car with a balanced/blueprinted/cammed/milled Sunbelt, special shocks from Mazda, etc.