I support the LED idea for ALL lights. I have been researching this for a while starting with night lighting in the house for very little power draw, then starting to target the car lol. In simple terms, Incondesent lights are my base point, fluorescent lights use about 1/4th the power to create the same amount of light, and LEDs use about 1/10th the power. So LEDs are nearly twice as efficent as fluorescent lights. The downside (or upside) of LEDs are that they are effected hugely on angle of light.
Check this out just to get an idea, there are tons of videos like this
One thing to point out, when you are hitting 1w+ LEDs you MUST keep them cool, under 160C. I have read the figure of 12 square inches per 3w led running at FULL power.
Another thin to point out, LEDs are most efficent at less power draw compaired to their max. Example is, my 3w LEDs i'm working with are rated 700ma @ 3.2-4vdc (forward voltage), but I'm planning to run mine around 350ma which will probably only loose about 25% brightness.
One final note on useing "raw" LEDs (DIY setups), you must have a curent limiting device, resisters make heat at higher powered LEDs, so a DC to DC converter is ideal since they are 85-90+% efficent.
The LED method should be a LOT cheaper than trying to add a second battery, probably somewhere in the ball park of $30 or so for the headlight conversion, the rest of the lights you could buy from the linke above me for plug and play (built in resisters).
To give a real world example, I'm running 4 small led lights in my bed room as a night light (on 24/7) at 7ma with 13.8v across the series (around 100mw draw + how inefficent the power supply is). It provides as much or slightly more than a typical 7w night light, but the light is spread out over the entire room instead of just by the wall. Remember, 7w lights are a warm light, and LEDs are a cold light, which skews perception of brightness and color.
Good luck in either way you go about your task!