My personal choice, would be to implement a constant current boost
converter, and return the LED to Vin. This requires high side current
measurement, but it allows the LED voltage drop to be less than the
battery voltage, and still protects the led in case of a driver
failure that would kill the LED in a buck mode circuit.

The boost-puck modules I found here implement that pretty nicely, and
can be driven by a simple voltage or PWM signal.
http://www.theledlight.com/luxeonled_drivers.html
In this project, I didn't have time to design and build, so I just
bought that module.
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