These aircraft are not airships, 40% of lift comes from the shape of the craft, as well as having directional fans. So the ballast thing is much less of an issue. I am guessing that they can move the shape of the ship to reduce the lift in cruise mode.
Darpa has also successfully demonstrated a system (known as COSH) which compresses and decompresses the Helium to reduce/ increase buoyancy.
Though I am sure in Kandahar they might be able to source some sort of heavy material, like earth, presume they have some of that there.
As for the wartime aspect, as fin1012 stated, you probably would not use it in contested airspace.