Ah! Gradebook histories is a setting I didn't try. (No users, no history in my mind I guess.) I'll try it and report back.
Thanks for the tip.
Edited to add: Apparently Moodle Restore thinks the same way I do. The option to restore Grade histories from a no-user backup is not available (X'd out on the 3. Settings step).
ETA2: Also tried this: When creating the backup, include enrolled users, and ADD Grade History (it's not selected by default). On restore, UN-select everything except for users (required) and Grade History. Still no manual items restored to Gradebook. (No items at all, in fact.) Fooey.
ETA3: May have found an odd bug. Using the same backup as last entry, restore only users, activities, and Grade History. UN-select all content except for the single activity. Restored course is completely blank -- not even the activity is present. In the Gradebook, no manual items restored, BUT there is an entry for the activity. Odd -- I would have expected the activity to be present in the course as well.
I'm abandoning this issue. It's probably already taken me more time to investigate than it would have taken the teacher to re-create the manual items in the gradebook.
(All this in Moodle v2.4.6)