Dropbox

In fact it works :

First duplicate "Dropbox.app" from mac to "/Users/frontrow/Applications/Dropbox.app" on appleTV

From ssh on appleTV

mkdir /Users/frontrow/Dropbox
ln -s /Users/frontrow/Applications/Dropbox.app /Applications/Dropbox.app
cd /Applications/Dropbox.app/Contents/MacOs
sudo Dropbox &

Dropbox will create the ".dropbox" folder with the "dropbox.db" which contains HOSTID (as shown in http://wiki.getdropbox.com/TipsAndTricks/TextBasedLinuxInstall)

you can also retrieve HOSTID just with this :

grep --binary-files=text -m1 "host_id" ~/.dropbox/dropbox.db | sed 's/.*host_id//' | openssl base64 -d | sed -n /V/p | sed s/V//

And finally to add your appleTV to dropbox you'll have to enter this url :

https://www.getdropbox.com/cli_link?host_id=HOSTID

in a web browser where HOSTID is the code we found previously.