I keep a copy of the federal statute with my registration. I'm a criminal defense paralegal for the Seattle/King County Public Defender, so I'm not shy when it comes to politely reminding a law enforcement officer that federal laws always trumps state and local laws.
If the LEO wants to escalate the encounter and attempt to cite me with obstruction (we call that "contempt of cop" in my business), I'll be happy to share the federal statute with the prosecuting attorney for the jurisdiction that doesn't like headlight modulators.
Having said all of that, I ALWAYS have my modulator on when riding between dawn and dusk, and I've ridden 75,000 miles with my headlight flashing - through big cities and remote small towns - and I've never had a LEO take issue with my flashing headlight.
Maybe it's because my flashing headlight is on the front of an all-white RTP...
I've attached a copy of the statute I carry in my radio box, kept dry in the Ziploc bag along with my registration and proof of insurance card.