Hi Mike: Thank you for the correction.
IMHO, the LED dot matrix is an assembled RGB LED matrix, so I think it could be of reference. And, could we see ratio of on to off time as the inverse of scan frequency? For the brightness, is it linear relation with duty rate too? I think human eyes might have some weakness distinguishing multiplexing trick... Not sure again, a controlled experiment is needed:(
We are testing the Matrix by this simple and rash way, taken assumption that the limitation of 500ma current limit would ensure the dot-matrix's safety.
Your suggestion on the current measurement is helpful, we would do it this way 
Lastly, apologize for not reading the post throughoutly, I read a comment on my flickr pic and it leads me here. My pleasure to join the discussion! ^^