It seems that Valmy (masculin in French, not feminin) was really used after the battle in 1792, when revolutionary names were usual (if some one is interested in a general review about some aspects of the revolutionary onomastics, I can look for the reference and put it here). The battle of Valmy was very present in people's minds:
"(25) Un certain nombre de batailles peuvent justifier ce prénom : Valmy, 22 septembre 1792 ; (...)
I don't know exactly when (if in 1792 immediately after the battle, in 1793 or later) and where (Paris, Reims...) the trend was started, but I found that Valmy Féaux (former Belgian minister) wrote a book about the subject: Valmy, bataille et prénom. The bond between the battle and the use of the name seems clear.
Perhaps someone in Belgium could check the book in a library.