On the back of a Teensy 3, I soldered one blue and one yellow-green extremely small surface mount LED between pins 27 and 28, and pins 29 and 30 respectively. I did not add external resistors. The T3 is clocked at 24Mhz and will be powered from an external 3.3v voltage regulator (an LP2950ACZ−3.3G), ideally sleeping most of the time.

By driving one pin of a pair high and the other pin low, the LEDs light up just fine with no apparent heating, tested while powered from my laptop's USB.

So far, I haven't tried leaving the LEDs on continuously. I'm guessing that by keeping the duty cycle of the LEDs low, I can avoid damage. Does anyone know if this is correct and how to calculate the maximum safe duty cycle?

I'm also wondering if the internal pullup/pulldown resistors could be used instead of external resistors to make a simpler safe approach?

By the way, in the intended application the T3 will be mounted directly against the project box with holes for the indicator lights, but based on my tests, it's tempting to make an ornament out of a T3 and a bunch of surface mount LEDs.