update: for some reason it worked once.
I found out that I had been disabling boot-from-CD in the BIOS, and I fixed that. The next time I tried booting it actually spun up the USB CD-ROM and loaded the welcome screen from EaseUS ToDo Backup software from the CD.
But then i was stuck. There was nothing I could do to navigate that screen. The cursor device on the edge of the LE1600 wasn't dong anything.
So I rebooted with my USB keyboard plugged back in and I've never been able to get it to boot off the CD since.
There must be some important magic I've left out. Any ideas?