Follow up: I ended up buying this HTC-branded car charger (currently $3 via Amazon). It properly charges my Galaxy Nexus in "AC" mode, not "USB" mode, without my needing to resort to any sort of surgery on the device. Amazon reviews on these sort of gadgets are often quite difficult to sort the wheat from the chaff. I ultimately figured that if the charger was branded from a non-Apple phone vendor, it would be more likely to follow the "standard" high-power spec rather than Apple's "non-standard" spec.