I got my ss20 to boot the install cdrom! I had 4x32mb memory modules in the machine. On a lark, I pulled two of them out, hoping to affect the memory allocation algorithm, and it worked.
If anybody else out there has an ss20 they've been unable to boot off cdrom, try pulling memory until it's got only 64mb, or perhaps anywhere than 128mb. I'm curious if my solution is reproducible. There's still a bug here, but this is a sufficient workaround for me. When I'm done installing, I'll put the memory back in. Marc