This: http://wlhdd.co.uk/wiki/Guide : would do the trick quite well - I have a couple... if only they had USB 2.0 (or 3...) but you can set it up as a Wifi AP or Wifi client and provide usb storage devices by CIFS/NFS/FTP etc. Should be able to do usb-over-ip too if you can compile the modules. Oh and it includes space for a 2.5" IDE drive... Could make a smaller case if you wanted to sacrifice that capability, or use the case space for a large battery pack.

Hmm thats a thought!!!!