Impossible. The path is hard coded in many places (`wp-admin/includes/update-core.php` for example).

I think this is worth a Trac ticket, if there isn’t one already. We can move the directories for plugins, themes and mu-plugins. This is the last thing why we need a wp-content directory at all.

### Update

There was a short discussion in [ticket #25153][1]. The result is a _wontfix_, because the `upgrade` directory enforces the existence of a `wp-content` directory (whatever its name is in your setup), and _that_ should be kept in case the core needs to add further directories here later, for example `wp-content/libraries/`.

You can set the `upload` directory and the `wp-content` directory to the same physical directory [via `wp-config.php` constants][2]. This should be clean enough for time being.


  [1]: http://core.trac.wordpress.org/ticket/25153
  [2]: http://codex.wordpress.org/Editing_wp-config.php#Moving_wp-content_folder