Well, I've had partial success. The latest firmware (v2.02AU.100722.TTS) allows my mac to see the two devices (sdcard and internal memory), says it needs initialising, so I partition and format to MS-DOS with Disk Utility, and hooray! it mounts up and I can ostensibly copy files to it.
Problems start when ejecting
If I drag to trash, it mounts up again within about 3 seconds.
If I use Disk Utility to Unmount, it goes away for good, but there are now no files visible, either within the reader, on OS X or windows (which just works...)
Ecoreader support has suggested that there is a config option somewhere that makes it work, but didn't know what that was, or what to ask apple support. I've not actually rung apple support yet, but their forums have not been helpful so far.
As most have suggested, it's easier to use an SDCard reader, which I'm doing now.