Well for that very high DPI printer I guess I could go to the printshop in the next town now that I think of it... trouble is getting exactly the right dimensions. We tried it on my dad's Canon 965i but that did not exactly work out.
Measure a real license plate and divide by 18"? *shrug*
The biggest problem I've seen is that just because you set the size of an image (in Photoshop, for example) it may still not print that exact same size. As Photoshop assigns the pixel count, the printer might interpret it's own pixel count and you end up with a different dimension at the final print. If nothing else I'd say make your image, and then resize it a few different times (start big and work your way down) and then just print all of them and pick the one that fits best.
Just a thought. Unfortunately for as much as I've THOUGHT about doing this, I've never done it. I started a 1/24th scale model of a '71 Road Runner, then bought a 1/24 Viper RT/10 and some polystyrene pieces and fit the Viper V10 into the Road Runner engine bay. I never finished it (still have it though, ready to resume at any time) and I was going to make "Road Runner GTS" decals for it, as well as some "Viper Powered" logos for the pop-up air grabber in the hood. Just... never finished it.