Thinking more on this - perhaps one of the in air data recorders like Eagle Tree will record voltage and current drawn. That relates to power used over time, which is another way to say mAh used. You could use this to get a reading of how much power is left.

Still it is easier to know you have a 1500 mAh pack and took 725 mAh out for a 7.5 minutes flight to figure out you have about 7.5 minutes of flight left. Though I would only fly an additional 5 minutes. LiPos do not like to be fully discharged! Pretty easy to do the math and a whole lot cheaper.
John