I've got to second the General on this one, how do you expect to do this? You can certainly do it while not plugged in by using a graphical sftp client or a sshfs mount, but if you specifically want to plug in, you'll either be wanting to mount the devices directly on the PC (as happens atm) or start up a gadget-ethernet connection and then do the same as you would over wifi?

There is a fundamental limitation with trying to masquerade as a block device, while something else is also using the device, so then you can't fool it into thinking the cards are mounted but also accessible on the IT. So then you're back to using file transfer over some network connection.