So it's possible that "dead" dies are actually not dead at all, but the solder below them has just melted and shorted across the array?
Yeah, basically. And since it is much more likely you shorted the +.6 supply it's not that uncommon.
The problem though is getting flux under the die for a reflow, without flux shorted balls will never "re-ball" themselves under heat. You could pull the die and attempt to re-ball but this is really getting towards the edge of what I can do (not impossible, just that placing the chip with a .1mm error is one ball off).
If I can find a way into the chip without being destructive I can at least remove an errant die, so we'll see. This is all to fix one guy's cube, I just don't seem to want to give up on this. :-)