The Vega program included a balloon floating in the Venus atmosphere. It seems no more balloons have been used on other planets since then. The Wikipedia article on Aerobots mentions JPL feasibility studies for Mars, Venus, Titan, and Jupiter, but does not cite any sources for Jupiter and the information there might be original research in the Wikipedia article.
Since then, have there been any official feasibility studies into descending balloons into the atmosphere of Jupiter or other outer planets? By official, I mean by or with funding from a major space agency or related institute, as opposed to semi-hobbyists doing calculations. What would be the biggest challenges, compared to the balloons that have been used on Venus?