I was thinking a very similar way on how to calibrate the LDR's, I have several high-grade multimeters with high accuracy on the Ohms.
The only problem for me is converting that data into Arduino code, but I suppose if you were to run the same experiment whilst changing the PWM output in the first place to change the LED brightness, then that would be doable, although slightly laborious since you have to upload new code with new values each time.
Also, this method doesn't transfer to other users, each person that builds one would have to go through the calibration stage. Whilst on a small scale this is no problem, I'm sure after we try it this way we could more than definatily find some other way.
Really like your profile idea as well, that's something I would have never of thought of. Great stuff.
Also, what method are you planning on changing the volume with, I was thinking rotary encoder since it's much easier to incorporate IR remote control than with a potentiometer. I haven't gave the rotary encoder tutorial much time but I remember him saying it would suffer when the steps were higher than 24 I think.