I think we can all agree that an airship working directly over enemy territory would be terribly vulnerable. Presumably, the ISIS would operate well behind a safety cordon of fighters the same way AWACS and JSTARS planes do.
I assume that the reason the DoD is exploring this option is the operational difficulty in maintaining multiple shifts of AWACS + JSTARS in a warzone. The airship, on the other hand, would be able to maintain its station for days on end, without the need for refueling or returning to base for a new crew.
However, a stickier problem emerges - would it be able to stay on station? According to this site: http://www.madsci.org/posts/archives/1999-09/938402041.Es.r.html
winds in the stratosphere are around 100mph. Would solar-powered engines be able to keep the airship in the same place in the sky?