Don't know if you saw this thread yet, but I was thinking about doing something similar, although a step behind what you are talking about.
On-the-Go Slope Measurement
Once I get it good to go, I might add in what you are talking about as I was wondering the same thing yesterday about P&G efficiency (specifically the fact that I have an automatic, so I have to glide in neutral not EOC, which I assume means that I have to glide for longer than a manual).