How to change employee Overtime Periods in bulk?

Summary: We need to update the Overtime Period for a few hundred employees. But we can't find which REST API endpoint to use, and none of our HDL attempts load successfully (import, yes; load, no).

Content: In the UI, we see the field on the Assignment Details section of the Responsive Employment Info page. The upper screenshot shows what we see for the typical, 40-hour work week employees; while the lower screenshot shows what we see for those with 24-day schedules. It seems to show the value from PER_ALL_ASSIGNMENTS_F.OVERTIME_PERIOD—and is empty when the value is null.