there is a way. If you can use a iso, you can probably also get other files on the system. Thus just make a USB flash drive with a bootable Ubuntu live system on another system. Then copy the created directories from the usb to your disk in a working installation and get the information you need for grub from the "syslinux/text.cfg" file that is created. If all goes to plan, your system should boot as if from the USB flash drive: it should find the filesystem.squashfs file and mound it with a loop. Just recreate the directory structure on the usb stick on your existing root directory.