FAR 119 covers what is considered a commercial operation. However, I believe 61.133 covers your priveledges and limitations with a commercial license. If you are doing something that neither excercises the priveledges of of your commercial ticket or falls under 119.1 as a commercial operation, I cant see as how it could be called commercial flight time.
I really believe that the real answer is the fact that you are not recieving compensation to fly. That would mean you are not using your commercial priveledges...just private.