This may seem obvious to some, but it wasn’t to me. I have spent the last number of hours on and off trawling bug reports and forums for the answer. Anyway, I’m not sure whether it makes any difference, but I am running Grub 2 as a coreboot payload. Perhaps that makes the command line incantation slightly different to the norm. Without further ado.
insmod ehci linux set root=(usb0) linux /isolinux/vmlinux root=live:/dev/sdb1 initrd /isolinux/initrd.img boot
Easy when you know how …