Back to some of the previous suggestions (this is a very interesting thread, BTW), I think you would want to prototype something with a 3-axis accelerometer, collect data for both 'flat' and 'toe' walking and see if you can find some patterns. Just trying to visualize it, I can imagine you will get acceleration (in a combination of axes) as the heel comes off the ground that you won't get (or to a lesser degree) if she is always on her toes.
Here is an Instructable for graphing acclerometer/gyro data in real time (uses Processing, a development environment not unlike the Arduino IDE...in fact the Arduino IDE was based on it), which would go a long way towards helping identify patterns (vs. just looking at raw data, which might be near impossible).