How can a PMM effectively approach product marketing when the primary focus is not on a tangible product, but rather on intangible factors such as safety and reputation management?

What I believe you are describing is selling a Service vs. a Product. Ultimately, it's the same thing.

For instance, a Marketing Agency's product is their marketing ideas, creative work, etc. but really, they provide a service.

Like any other product, for a service, you should know the target buyer, the challenge you are solving for them, and how you do it most effectively. You should have a good understanding of the competition and the overall market and industry.

Make sure you're clear about what it is you are actually selling, but still lead in with the value of safety and reputation management.