Pretty much what I was thinking of doing, for example my system is going to be a Brutus 20 electric (controlled by an Arduino/Android): Here would be the steps in theory:
1. Pre mash water volume in MT - calculated based on force gauge and known density of water, zero out the weight of the equipment (use water value for recipe based on calculations from beersmith or however)
2. Sparge water volume in BK - calculated same as MT and the amount via a program or however
3. After mash and recirc and all wort is transfered to BK measure SG, based on SG can find density and get preboil volume from force guage and calculations.
4. Adjust water volume as needed to get desired volume if low, this is again easy, you know the current weight and can figure out how much water to add to get desired volume
5. Post boil volume, same as pre boil can take SG reading and with the weight know the volume.
Here are the problems I see with this technique: If you are wanting to boil off to a specific volume would need to know the starting volume and SG and could only calculate a theoretical final SG based on a boil off calculator and then could find a theoretical weight. This can be useful to know in cases where your brewing environment changes, for example, outside humidity and temp changes. You could tell if your boil off rate is too vigorous and need to lower heat or cover to adjust.