Apparently you can get it to appear by running Steam with -dev commandline arg then using the install_folder_ui command. It's in the beta client update. Used space sorts horribly though. It sorts by text rather than value.
So it's obviously still in testing but it appears to be a major feature coming soon.
This comment was edited on Sep 11, 2012, 09:51.
Playing: Dark Souls 2: Scholars of the First Sin Watching: Mr. Robot