i needed to do this and I think I ran into permission problems as well.

 

the solution I resorted to was , uploading the file to a temp folder in the same domain,  /uploadtemp  ( whatever ).

 

I set up ftp access to the folder i needed ,  in the call back function of the uploader, called a php page to ftp the files to the other server, then unlinking the file in  the /uploadtemp dir.

 

might be a bit of a round about way , but the user is unaware of whats actually happening anyway

 

hope that helps