What about the charger adapter?
Did you tear apart the charger adapter as well? There's at least a little more going on there than simply applying USB voltage to the Flex.
From what I could tell, the center pin is +5 VDC with respect to each of the two end pins (which are at 0 VDC wrt each other), and about 100 Kohm to each of them (with 200 Kohm between the two ends).
It seems probable that, in a pinch, you can charge your Flex by putting 5 VDC across center and one (or the other?) of the end pins. But the adapter also has the ability to apply a reset signal (didn't measure it) to the Flex when you press the inset button.
Any interest in tearing the adapter apart, as well?