The idea is correct but the implementation would be rather difficult. You also have to consider how much fuel you would actually save. On one hour flights in my aircraft you'd save approx. 12 kgs/1,000 kgs but as the passenger weights are a reasonable average, little would be saved but the cost of gathering the data would cost more than the fuel wasted. However, a one minute flight reduction saves 30 kgs. A pure idle descent saves about 150 kgs. A continuous climb saves even more. These are the areas where fuel savings can be made. Also, rather surprisingly, it's "environmental" and political restrictions that really piss away fuel. Pointless noise restrictions such as a minimum of seven mile finals when visual, SIDs which take you all round the houses, routings to avoid unused airspace because "telephone calls take up too much time" and the such like are the wasters of fuel.
However, you might be able to save some fuel by optimal CofG planning - only so long as you don't allow people to select their seats, which is what the full service airlines offer.
PM