LEDs are fun. I just wrapped up the rear light controller. Works fine now. It is so bright that I had to turn down the overlayed flash because it would really annoy drivers when it gets dark. So now it's just a barely noticeable extra flash, should be enough to make people wonder "What the heck is it way down the road there?".
The perception of brightness difference (in this case flashing) is surprisingly small from 100mA to 300mA.