Only if there is plenty of room for it to float. It's unlikely that you would be able to get an accurate reading, though maybe if you pushed it down with the airlock and removed that to let it float up, it might work. Two obvious problems that I see are the likelihood that kausening residue will be all over it and make it impossible to read and the possiblity that it could float up and clog the airlock outlet would make a blowout very likely.
In all, not a great idea. You could maybe drop it in during secondary and leave it there until racking for bottling.