Three things...
1. Manual entry of workgroup (non-domain) machines on the management server seems to work: as long as you only use the Windows computer name (no domain suffix) for the computer name. I've used uppercase for the ID, and lowercase for the DNS entry; it might be case-sensitive.
2. The non-domain systems need to be set to use a non-domain IP/hostname to use for contacting the management server (change made in Settings->Troubleshooting->Admin Console).
3. An unexpected bug: one of my non-domain XP systems has an underscore in its computer name, but the management server won't allow underscore DNS entries.