Interesting thought...but i doubt it would be measurably effective.
Our bodies simply don't provide enough feedback, other then gross weight - and even that is subjective. 30 minutes of sustained Target Heart Rate aerobics is burning hundreds of calories...which equates out to ounces of adipose (body fat) reduction.
Cardio is first and foremost heart/lung conditioning. If your working within your Targeted heart rate, then up to 85% of the energy required to sustain it is coming from adipose tissue since glycogen stores have all but been exhausted. Also, the number of total calories would be radically different from one person to the next. For example, someone that is light weight and 'aerobically fit' would consume less calories then someone who isn't. Since there is no metric or baseline for either - how would you know your getting enough calories, or too much? But again... The difference is in ounces. Eating them before or pulling from body fat is the same net result.
Personally I don't believe micro-managing calories is even possible. But i could be wrong.