Hi
The main problem with trying to modulate our Cree lights is the in built driver circuits for the LED 'fight' any modulation and almost stop the effect all-together. This would also go for any active dimmer circuit, the lights would remain fully lit until they reached a point and then would just stop working.
Even if you did manage to get a modulation effect, with 900 lumens of light it is going to be akin to a strobe, which would certainly get you in a lot of trouble with the local law enforcement.

Sorry to not be of much help
Regards