Offline copy with SOAP and a client side Java app
A sensible architecture for this would seem to me to be a client side Java app that communicates with the server through SOAP to create an offline cache since last sync.
AJAX is never going to provide client side persistence without a lot of headaches I would have thought.
Take a look at thunderbird for offline ideas
The real problem for offline storage lies in permissions for writing via a browser to a local file. We have done quite a bit of XUL and web app development, I but I (personally) have never done any kind of web app that needs to write to a file.
I'm sure others can tell you if that is possible.