What about a system like AO3 (Archive Of Our Own), where the content alerts are tags, and "Creator Chooses Not to Use Content Warnings" is an option? That would be backwards-compatible, for the current database, where creators could then go in and update records. The feasibility of this does of course depend on how your database is set up.
AO3 also by default has a "Hey, might have adult content!" warning page that pops up before you can even access fanfic not specifically marked as okay, unless you have specifically set that it's okay.