The following script will act according to your desciption. I believe this is not the best approach for this situation, but there is not an applet with the functionality described by you that I know of.
Create a file in your home folder with the following content:
if [ $# -eq 0 ] ;
for file in "$@" ;
echo $(($i * 100 / $#))
cp "$file" "$ubuntuone_loc"
)|zenity --progress --title="Copy files progress" --text="Copying $# files to $ubuntuone_loc..." --auto-close &
while [ $RUNNING -eq 0 ]
if [ -z "$(pidof zenity)" ]
kill $(pidof cp)
Make sure you replace "/home/user/Ubuntu One" with your ubuntu one folder. Now create a new custom launcher, and fill in "sh /path/to/the/created/file". The new launcher should work with clicking and drag and drop.
I hope this works for you. A custom Ubuntu One applet with this sort of behaviour would be nice.