This is spot on and how it was where I grew up and was a volunteer firefighter. IIRC, the visible strobes on our equipment we referred to as "opticoms"

Opticoms can be preempted with a strobe light using an IR filter; they don't need to receive visible light to activate.

If the Opticom sequence is trivial, the hardware is easy. If its a complex pattern, a 555 timer is required.

Sitting outside your local fire station recording video of the strobe pattern on an emergency vehicle should be sufficient to determine the necessary pattern (you step through frame by frame to identify the sequence).

The above is informational only. One should keep in mind that today, most traffic lights will log the preemption with their central controller, and that unauthorized use is illegal in most jurisdictions.

I doubt the relays that control the headlights are even capable of switching at the required frequency...