Changing fstab during installation

If installing FreeBSD from a USB stick onto a USB hard disk the target of the installation is da1. On reboot the USB stick is removed and the USB disk becomes da0, which result in a mountroot problem because of an incorrect fstab. What is the best way of getting around this problem? I realise I can use vi to edit fstabbefore rebooting, but I'm not sure I can add labels to the partitions when creating GPT partitions... Did I miss something?
When the installer reaches the point where disk partitions are created, there is a "Shell" option you can use to manually create partitions with proper labels and create a custom fstab.