As far as I know you cannot have any modulating lights. I wanted to do something like this for when I sound the horn but was warned against it:
[video=youtube;zjf0ytJ-T7s]https://www.youtube.com/watch?v=zjf0ytJ-T7s[/video]
Modulating headlights are specifically allowed on motorcycles (with a decently defined set of rules on their operation). Flashing lights are prohibited.
It looks like you could wire a modulator into your horn circuit without running afoul of the law (assuming it met all of the technical requirements).
Obviously, that bike as shown fails for other reasons (at least 6 bright lights). Hopefully the quantity of lights gets removed the next time the standard gets revised, that was always a stupid metric. It would make much more sense to provide a maximum light intensity and let people figure out the best way to achieve it. Are they considering the 6' wide LED bars as a single light or 150 lights? In either case, they are horrendous and shouldn't be turned on on a public road.
Technical Standards Document No. 108, Revision 4 — Lamps, Reflective Devices, and Associated Equipment
See this link: Section S7.9 Motorcycles
Effective Date: June 9, 2007
Mandatory Compliance Date: December 9, 2007
S7.9.4 Motorcycle headlamp modulation system
S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the upper beam or the lower beam from its maximum intensity to a lesser intensity, provided that:
(a) The rate of modulation shall be 240 ± 40 cycles per minute.
(b) The headlamp shall be operated at maximum power for 50 to 70 percent of each cycle.
(c) The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity measured at the same point.
(d) The modulator switch shall be wired in the power lead of the beam filament being modulated and not in the ground side of the circuit.
(e) Means shall be provided so that both the lower beam and upper beam remain operable in the event of a modulator failure.
(f) The system shall include a sensor mounted with the axis of its sensing element perpendicular to a horizontal plane. Headlamp modulation shall cease whenever the level of light emitted by a tungsten filament light operating at 3000 degrees Kelvin is either less than 270 lux (25 foot-candles) of direct light for upward pointing sensors or less than 60 lux (5.6 foot-candles) of reflected light for downward pointing sensors. The light is measured by a silicon cell type light meter that is located at the sensor and pointing in the same direction as the sensor. A Kodak Gray Card (Kodak R-27) is placed at ground level to simulate the road surface in testing downward pointing sensors.
(g) When tested in accordance with the test profile shown in Figure 9, the voltage drop across the modulator when the lamp is on at all test conditions for 12 volt systems and 6 volt systems shall not be greater than 0.45 volt. The modulator shall meet all the provisions of this TSD the standard after completion of the test profile shown in Figure 9.
(h) Means shall be provided so that both the lower and upper beam function at design voltage when the headlamp control switch is in either the lower or upper beam position when the modulator is off.