You have to change the domain statement as you have done (this is a pain!)
and you should be able to use the "Backupregistry" option.

Below is from a 4.2 manual, I assume 5.1 is similar:

Backupregistry

The backupregistry option specifies whether to back up the Windows registry
during domain incremental backup or backups which include the Windows system
drive.

On the native and Web GUIs, this option is only valid when executing the
Backup Domain action.

Note:
For Windows 2000, this option is ignored if you specify systemobject in a
domain statement during an incremental backup.
Syntax

                  .-Yes-.
>>-BACKUPRegistry-+-----+--------------------------------------><
                  '-No--'


Tim Rushforth
City of Winnipeg