Last night before bed I whipped up a circuit simulation of the 5V regulator using iCircuit running on OS X. It’s a little buggy, and it’s certainly no Spice, but the real-time feedback is fantastic.
The beauty of this is I can play with it and examine possible failure scenarios by shorting or opening various components to see what happens. I think I have a much better idea of how the regulator actually works, now, and I’m going to do a careful part-by-part check, from one end of the circuit to the other, and find out what’s failed. My gut tells me I should check R50 (the potentiometer), R47, and the 2.4V reference zener. So I’m not giving up hope yet!