Thanks. It doesn't help me at the moment, but it will probably help somebody else who has that model and the same question.
I tested the 8800 some more and found conductivity from the bottom contact to ground on the USB port, so I went with that and confirmed empirically that the top contact is positive and the bottom contact is negative. There is a reverse diode between them, so if you connect things backwards, I believe it will short the supply and (theoretically) not damage the phone. I didn't try that, though, since I just paid full price for this phone last week.