Got the SS20 working!! (woo-hoo!)
Downloaded the tftpboot.img file fom phunnypharm.org, netbooted, and the
installer is running as I write this! I'm afraid the kernel images in
r2 for the sun4m architecture may not be kosher - maybe someone with
more Debian experience than I can verify this.
Thanks for the assistance, everyone. At least I got to learn how to set
up a boot server :)