-enough information that a USB driver can be written for non-Windows

This isn't very likely, at least with the current firmware, because it wouldn't be very useful -- it's a read/write sector kind of interface, which means the PC needs to implement the filesystem, and Karma uses an extremely proprietary and icky filesystem. Non-Windows is what the Ethernet interface is for.

IMO (but there are other opinions at Rio) the best answer would be a so-called "mass storage class" player, one where you plug it in and with no drivers at all it becomes another drive letter, like Ipods and digital cameras do, due to having a standard Fat32 filesystem.

Peter