I would like to boot from an external USB CD/DVD drive on a Toshiba Portege which can't. I've tried with plop, but haven't had any luck.
I'm sorry, but your only options seems to be plop. I have successfully tried on a computer whose BIOS that doesn't allow booting from USB-CD-Drive.
GRUB2 itself doesn't support booting from a CD-Drive but it supports booting from ISO files.