I just started building the second Ifos prototype from scratch, and already I've discovered an issue that problably contributed to the DOA-ness of assembly attempt #1. It turns out the land pattern for the voltage regulator's heatsink doesn't quite match the heatsink I'm actually using. When installed, the heatsink just barely clears the solder pads, and the mounting nut allows enough play that the pads may actually be shorted together.
The immediate fix was to simply cut back the heatsink with a hacksaw. This allowed enough clearance to prevent the short-circuit.