Do you have a true-RMS meter? It's also possible that the average voltage really is 450mV, but it is fed in pulses of greater magnitude.
It's been a while since I reverse engineered the remote for my door opener but IIRC it uses AC, I don't recall whether it's 60Hz from the power line or something else, but by clever use of diodes it's able to light the LED and differentiate between three separate buttons using only 2 wires to the remote unit and no fancy digital interface.
Something to keep in mind with the mcd ratings of LEDs, they are affected greatly by beam angle. An LED with a really narrow beam will have a much higher mcd rating than the same LED chip with a diffuse lens but it's the same amount of light. It will look brighter if you look directly at it but off axis it will be dimmer. If you want max brightness and cannot change the drive current, look at some of the high efficiency green LEDs, they're right near the peak sensitivity of the human eye and can appear quite bright on just a few mA.