My suspicion is that this is down to badly made car chargers, and on a previous occasion using the Testing menu, I noticed that my own charger was being reported by the phone as USB.
So I set about trying to rectify this, using my awesome () knowledge of how the phone recognises what it's being charged with and an 'I don't care if I kill the charger, it was free' attitude.
I took it apart and tested continuity between the 2 middle (data) pins on the USB plug, there was none, and the problem was identified. I simply flipped the board over and soldered these 2 pins together and reassembled the charger.
Took it out to the car, plugged it in and lo and behold, the Testing menu now reports that it is plugged into an AC source! As my phone is fully charged and I don't have any trips planned right now I don't know how much real world difference there is, but it's looking promising, at least the phone now knows it can draw 1000mA instead of 500mA, and for 5 minutes 'work' it was worth doing.
I didn't bother with a how-to as I figured chances of someone having the same charger as me were slim, but pics aren't a problem:
Here is my universal USB charger with the cover removed:
and the reverse side with the 2 pins soldered together:
Probably doesn't tell you a huge amount unfortunately, but to be honest that's because it really is a simple procedure with not much to tell.