I only got a few minutes to try to put a resistor between the regulator and the Led. I was hoping it was just using a voltage source but it turned out to be a current source. Putting a resistor in the circuit caused the input power to increase and the current through the led to be the same. That is no change in brightness. The problem here is the input current is 1.6 amps and the current through the LEDs is 2.6 amps. (20 watts) To dim the leds I would have to put the resistor in parallel with the LED to siphon off the power. This would be very power wasteful and would create a lot of heat in the added shunt resistor (15 watts). Time for more surgery...
The regulator is potted with a black sticky goo. I will see if I can clean it out and see if I can find the feedback resistor in the current control loop... It is just a simple buck switching regulator... (the best solution)
I will also measure the switching frequency and check the startup and hold up times to see if we could just pulse width modulate the input power. (the sloppy solution)
Sorry for the tech talk, - it is part of what I do for a living...
Now where is my big hammer......