Fantastic thread. Not quite trench art but the other day I saw a WWI-era (well, to be fair it could've been from any time from the mid-19th century to WWII) French military kepi with oak leaf embroidery (bit like
this) in an antiquities store, when I went in to see if they had any coins for sale. Out of my price range, but a piece of history nonetheless.
The only WWI items I have (aside from coins/notes/stamps from the war years) are military medals; specifically the British 1914-18 medal in silver, a British (well, NZEF) 1914-15 star, the British and French Inter-Allied victory medals, and an Austro-Hungarian medal (that I found in a flea market outside Paris of all places).
I also have this Field Service postcard from 1918:
There have been a couple of pennies from the years preceding WWI with military numbers on them on the ID forum recently, I'll share one here:
https://en.numista.com/forum/topic67597.html