This is a cool idea. You would probably want to run the PCCaaS as a non-profit and donate some of the money (5% seems generous?) to appropriate causes. The main API would be for displaying an appropriate banner of course.

Another API would be to determine if a specific domain subscribes to the service and how much they care about the appropriate topic (in terms of "points" which are partly correlated with how much they spend on the PCCaaS, but also with some human input). This would be useful to people looking to vote with their wallet. I bet there are plenty of artists who would love to make custom banners, kind of like Google's doodle of the day.

A third API would be to get notified when a customer downgrades or terminates their plan with the Protest Current Thing as a Service. Journalists could subscribe to this last API to get ideas for news stories. /s

That's a great idea. The only problem would be determining the correct set of things to be protested at any given time. So I'd suggest grouping them into flavors - say US liberal or US conservative flavors. You just choose the one you subscribe to and then let the service decide whether to insert say BLM or anti-CRT messages at any given time.

NPC as a service.