I cant say that dirigibles were used in WW1, but I know for a fact that they were used during the American Civil War.
They were used as reconnaissance balloons, to overlook battle fields and for oncoming military formations. It would SEEM, that with the advent of fixed winged aircraft, the use of lighter than air craft would have ceased, but it seems that they were still used all the way up to and including WW2, but with different tactical purposes in mind.
Most of the dirigibles used during WW2 were used as anti-aircraft defenses. The mooring cables would do a real number on any aircraft that ran into it, ere the deterent to bombers and fighters alike.
But I am willing to bet, that with trench warfare being the most common form of defense, and the still then common place mass frontal attack employed as the primary form of advance, that the hot air ballons were still being used for much the same purpose as during the American Civil war.
I could be wrong, but that is my opinion.
So SHOULD you use balloons in your game? I would enjoy seeing them there.
One question, where can I get a copy of your game ? Is it available for download ?