These are the LEDs I'm using: http://www.ledssuperbright.com/3w-led-c-11/3w-high-power-led-aluminum-wide-angle-module-p-218
They have that little inline transformer there that takes 12v and steps it down to 4v, but it also negates PWM control of the speed that I need. The website says that removing the tranformer lets the LED run at 4v, so that's what I did. The aluminum housing keeps the LEDs plenty cool; I've run then for longer periods of time with no noticeable warmth (The brake/running light will be on whenever the bike is on, but dimmed until the brake is used)
OK, they look like they're attached to a decent chunk of metal and they're running at design power. Should be fine.
Oh, they do them in UV as well. I've been looking for something like that...