Corey,
Here is a hack at making a linear constant-current regulator for a string of three of the W724C0 LEDs using my favorite LM431. I configured it to deliver ~1.6A. Note that it keeps the LED current constant while the battery voltage sweeps from 12.2 to 15V. Below 12.2V, the NFET is saturated, and the current decreases. R1 sets the current.
In the third plot pane, see the power dissipation in one of the LEDs, the FET, and the resistor R1. At 14.5V (motor running alternator voltage), the dissipation in the FET and R1 are about 4W each; quite manageable.
Let me know if this seems to be heading in the right direction. If it is, when I get some more time, I will simulate the effects of transients.