You can buy a "Canbus module" for them that fixes the problem. You're right, it's the low wattage that causes the car to think the bulb's blown, but the "Canbus module" is nothing more than a H4 male, H4 female socket with some wire and a resistor pack to up the load which is self-defeating. I've seen some priced at $40 or more, which is ridiculous when you know what they are.
I am looking at buying a couple of different LED H4 globes. My bike takes 2x H4 globes as well as my car, so I thought I'd try a couple of different packs and see what happens. The bike doesn't have a canbus anyway (no OBD) and the car (2009) doesn't care. Will be interesting to see if there's any improvement in light output, but also importantly, in the amount of glare for oncoming drivers.