Today, Valve announced an update to the Steam beta client that allows you to download unreleased games and install them to any folder you'd like. Previously, this only worked for games that were already released.
Preloaded games — or games that you can download to your machine in anticipation of their release — could previously only be installed in the primary home folder for Steam, even if you had set up install locations for other games elsewhere. Now, even preloaded games can be sent to any location you choose. You need the Steam beta client in order to use this feature, which can be enabled under Settings > Account.