Why not build a capacitance type fuel indicator like used in fullscale jets. Tank probes are needed. What happens is you have 2 die-electrics. Fuel and air. As fuel goes down, capacitance changes due to the change in die-electrics. Make a trigger circuit to alert you somehow when a certain capacitance is achieved.

Complicated? yes. tedious? yes. cool? yes. practical? not for RC applications.
My TX has a timer that turns on when I advance the throttle. I havent run out of fuel in flight in a long time.

Good luck!
Sean