About time I got around to commenting on this one, I have a couple pieces of wood that I've aquakneadited to some flat rocks. I didn't do it to keep them from floating, in my case it was more because they were basically long sticks and I wanted them to stay upright reaching from the bottom to the top of the tank. Stupid fish constantly knock them over otherwise.
But, it seems to be working well, and the same thing would work for sinking pieces of driftwood. I actually suspect aquakneaditing dry wood would work better than wet like I did it. Wet is more flexible and squishy so doesn't get held in place as tightly by the kneadit.