The only trouble with something like that is you're essentially paying "too much" for your application. Since the Arduino already has its own regulator, you don't need such tight tolerance on the output. They can also supply a lot more current than you would ever need if you're powering the daughter board separately.
Where do you plan on locating it? You could look at cigarette lighter USB chargers. You could get a couple of them from eBay for $5 and if need be you can hack one up to wire it directly instead of having to plug it in inside the car.