I think you hit the nail on the head. Sure, carry a USB, card, tattoo, whatever. Personally, there's no way I'm going to let you have your heart attack/bleed out/stop breathing while I'm rifling through your pockets checking every electronic doo-dads there. I'm starting treatment.

If I see a bracelet, necklace, or tattoo (not my first choice, cuz I don't always undress a patient), I'll read it, cuz it's quick, simple, easily seen, and most importantly, easily accessible.