I don't think the TD needs protection, as long as the available bias current can't get too big. Once past the peak current, the TD will switch to valley current and voltage, and should be just fine. The question then is what happens past the valley - how much current and voltage are available to continue up the diode curve, where power dissipation increases a lot. It depends on the specifics of the circuitry, which could be figured out if necessary.
I read the circuit description, and I can see how it may be possible for the bias current to get relatively quite large if things get out of control. The circuit is a sort of auto-leveling system, testing the TD during each cycle, and adjusting the arming current according to its characteristics - pretty slick, I think. The bias generator is an amplifier capable of more current than needed for operation, and its output is controlled by the TD state comparator, in kind of a sample-hold setup, with a simple long-tail pair.
If the -12.4 V supply is lost, the comparator/S-H stops working, and the amplifier goes open-loop, to whatever maximum current it can deliver, possibly risking overheating the TD. In this event, Q32 forces the current into ground instead.
Ed
