From: Bremen, Germany
In a way one could argue that this is included in infantry and artillery techs.
One solution could be to offer "one-time" tech advances, to be used in a single combat of choice.
Ted Raicers "The Great War in Europe" used a chit-pull procedure to offer players some chemcal weapons and flame throwers. If the player was lucky to pull such a chit during the chit pull phase, he could use it to enhance one combat result. But only as long as he was the first player to use this specific gas, like chlorine. Once your opponent used "his" chlorine first, your own chit became worthless, as these weapons only offered a short time bonus, after which all war parties adjusted their forces accordingly.
I like this chit pull phase very much, it was a brilliant way to enhance the game play.
"You will be dead, so long as you refuse to die" (George MacDonald)