So I made some inquiries and I was incorrect, the rapid flash rear brake light functionality is offered by the OEM.
This functionality would be programmed into the controller and the the PIU would be the arbitrator of the lighting functionality.
So perhaps the ForScan suggestion was appropriate.
This feature only meets FMVSS when the vehicle's regular OEM functionality is programmed to override the feature.
Apparently some of the aftermarket modules don't allow the vehicles OEM functionality to be primarily so they actually violate FMVSS
You learn something new everyday!