There should not be any issue with a current source configuration on an LM317, however you are going to have significant heat to dissipate with 6 volts of drop from 12v with 750 mA. With greater then 4.5 watts of heat to dissipate it will require a reasonable sized heat sink for any linear current source regulator. With an auto running between 12v and 14.5v just a 9.4 ohm dropping power resistor will only have a +/- 9.4% current variation on the 750 mA nominal.
More complicated but have you considered an LED current sourcing switching regulator?