Can you still set the tmp_dir? I looked at a tutorial where you could but it was mighty old. Anyway if you can...
When you talked to your host and said a temp directory they probably though one of the servers stupid Temporary_Files folders or something like that. The reason your probably coudn't do that is you don't have acces to it, but if you set the tmp_dir to a folder you do have access to such as any folder you seen in your ftp when you connect to it you should be fine.
So try setting the tmp_dir to the folder you want the file in and then just use the move command and move it into that folder. Tell me if it works too.
Meh 2 cents is in!