I found a solution to this 'problem' now.. After the base image is written to the USB drive, you can press 'esc' before it boots.. that will allow you to set boot options.
You can then launch it with: chromeos-usb.A root=/dev/sdb3 cros_efi cros_debug
(assuming that /dev/sdb3 is the rootfs partition of your USB drive)
that will give you ctrl+alt+f2 and shell access and will allow you to change the password with chromeos-setdevpasswd.. after that is done > ctrl+alt+f1, ctrl+alt+t and launch 'install' with your newly created password
This will install the base image to your HDD.. please note that the base image launched with cros_secure and without developer mode by default, so the test images are probably a better choice.. or you can launch the base image from the usb stick again with the same commandline as before, and edit the grub.cfg so that it boots in dev mode.