Use symbolic links pointing to a directory outside the application tree? This is of course assuming that Media Temple uses *NIX servers. Currently that's how I manage certain subdirectories in my public directory, albeit for a different reason.
Symbolic links are a feature in *NIX filesystems. A way you could use it for the problem you have is to create a directory, say "uploads", somewhere above the Rails directory. Let's say the full path to this directory is /home/username/uploads, and your Rails application lives at /home/username/railsapp.
In the /home/username/railsapp/public directory, you would then create a link called uploads, pointing to /home/username/uploads. This means that when you access /home/username/railsapp/public/uploads, it actually maps to /home/username/uploads. Follow me?
That way, you have a directory higher up in the filesystem hierarchy that is still accessible by the webserver.
If you have SSH access, the *NIX command to create a symlink is "ln -s /the/path/to/link/to [name]". If you omit the name parameter, it defaults to the last component of the path argument (in this contrived example, that would be "to"). If you don't have shell access, you'd have to find out how to create a symlink via a web interface or something similar. I don' t know if it's possible to create them via FTP.
I hope I've made myself clear enough. If not, just yell at me.
~P
