so i figured out the statue tilesets are referred to in the creature graphics folder and have the token: STATUE_CREATURE_GRAPHICS
I dont see this referred to elsewhere un the files yet so the use of it might be hardcoded, which means adding new statues would be limited to stuff using that token.
So first step i guess is testing adding graphics to creatures that exist as statue types but dont have an assiciated statue graphic
I do wonder if making an entry under the plant graphics folder for STATUE_PLANT_GRAPHICS would work? Could also try adding a plant statue under the CREATURE_GRAPHICS tag tho im a little concerned it would then show up under creatures and not plants when specifying statue images in game

I too would like to know more about this. I also want to edit/add to the list of engraving descriptors