I think it might work to dangle a float into the center of the fermenter on a piece of fishing line. This would require being able to measure the very small downward force, to calculate the buoyancy change. Ideally, I would be able to read the gravity with a microcontroller, but a mechanical/visual scale would also be sufficient.
I can't think of a sensor that would work for this and also be usable.
Maybe it would work to hook the fishing line to a long lever to multiply the force and then a standard strain gauge would be able to register.