Hi Craig,
My utility tracks something similar for our assets; because Maximo doesn’t have a separate field for the certification rather than the completion, we added a custom field to the asset table for the “Renewal Date” in our case. One option could be that you add a field to the Asset table for the certification date, then add an escalation that runs nightly to update the last completed date on the PM to match the certification date of the asset so that a floating schedule will be in line with that rather than the actfinish date. If you needed a full history of the certification dates you could create a child object on the asset rather than just a field for the most recent certification date.