Windows Server 2012 in combination with Windows 8 will support something that is called Dynamic In-Session USB Redirection for Session-Based Desktop deployments as well as Virtual Machine-Based Desktop deployments. This will allow you to select a USB device for redirection during a session!
More info on this an other features of the new Remote FX Protocol here: http://blogs.msdn.com/b/rds/archive/2012/09/11/remotefx-usb-redirection-in-windows-server-2012-and-windows-8.aspx