Hmm... I'm curious, how does the capacitance sensor measure density? Would it work in the air?
a) they measure the total capacitance of all material in proximity to the sensor electrode. this material's capacitance is determined by its dielectric constant multiplied by how much of it there is. since wort is heavier at the start of fermentation than it is at the end, i'd guess that its dielectric constant changes too. that's why i suggested a capacitnce sensor.
b) yep, they will work through air quite well, although quantum for some reason stress that that isn't their designed function. i've used them to build a theremin, to quite good effect. it's worth bearing in mind that the design of the sensor electrode ("antenna") has a large bearing on the performance of the system. in the case of my theremin, i had to use a system of antennas (actually a slightly modded TV rabbit-ear setup) to increase the sensing distance to beyond about 300mm. obviously, this won't be an issue for the wort density sensor.
s: you might need to be careful about bubbles sticking to the electrode...? also, keep in mind that the the qt300 isn't good for single-shot measurements - the first few values returned are always inaccurate. the trick is to request about 100 readings in rapid succession, and use the average (or sum) of the last 20 or so as your data.