Hourly Rate Augments for Step & Grade Rate

Hi all.  I've posted this before a few years ago with no valid resolution. FLSA states that any discretionary pay must be included in the calculation of overtime. I've worked with several public sector clients who interpret this (and there are court cases supporting) that any additional pay, like Longevity, must be included in the calculation of overtime pay. Longevity is paid in lump sum in December each year.  Where employees are paid via PR16 Step & Grade, we cannot use HR14 Alternate Rate to resolve. We need PR16 because shift pay is set up on PR24 and is tied to PR16 S&G schedules. I've tried using PR30 for the work period equivalent, but that won't work because Longevity should only be included for overtime pay codes.  I've tried various settings on PR20.2 Pay Summary Group overtime flags, but hasn't worked so far.  Any creative ideas out there?