Currently I'm using /up as the default upload folder. It's the one and only folder that's currently writable and according to my company policies, it'll remain so.
I'm trying to use /up as the working folder for W3TC, so I had my share of tinkering with define.php, and with the 3 php files that get copied from lib/W3/Plugin to the working folder.
I haven't been able to make it work, though. In fact, it seems that I've managed to erase most of my development installation content.
Haven't you considered leaving the working folder or path for the user to choose? As I told you, there's no way in hell our platform engineers would agree giving the folder wp-content a 777 permision.