The airplane was invented in 1903 and was just beginning to be used by the military when WWI started in 1914. There are reservist steins for Luftschiffer (Air ship) battalions and Flieger (Aviation) battalions. These reservist steins would be rarer than those found for the infantry, artillery and cavalry units.
Also, they are more likely to feature balloon or zeppelin than an airplane. The latest German Regimental Beer Steins book, Deutsche Reservisten-Bierkruege, features several of these steins.