I got the card running again with the volume serial number 'AA52-6922'. I guess that's how the Eye-Fi manager actually finds the card, rather than looking for the EyeFi/r* files like my software has been. Anybody know offhand how to find this stuff in Linux?
I also got a request to make my manager software work on OSX. I'd love to do that, but I don't have a Mac. If anyone wants to give me ssh access to one or something, I'd be happy to give it a shot. Shouldn't be more than an hour or two of work, as long as there are development tools around that I can use.