You can measure gravity from inside the wort, not at the top. Put a piece of bouyant (i.e., sp.gr. < 1) on an arm and measure the angle that it hangs at. A little accelerometer device (I like the ST Micro devices) can easily measure this.
The device needs a level bubble to be held exactly level inside. Hold it inside with a magnet on the outside.
Transmit the gravity with RF (lots of simple small solutions there) to a display on the outside. Maybe the "holding" magnet and the simple display are on the same assy.
I use something similar for my saltwater fish tank.