This is the principle upon which the Galileo thermometer works. The problem is, as with the thermometer, the temperature of your mead will affect the accuracy of the balls. Just like your run of the mill hydrometer, if it's not at the calibration temperature, the readings will be off.
within the normal lager-ale fermentation temperature the density of water does not change much with temperature - it drops about 0.4% from 5C to 30C.
You need to correct for thermal expansion when temperature is high - like well above >100F, below 80F it's fairly flat.