I'm not sure if I was running it properly, but when I customize a weapon, I can't test its properties and there doesn't appear to be a way to assign a sprite to the weapon. It's a cool mod in concept though!
Viewing post in Nuclear Throne Together Weapon Maker comments
Sorry for the late reply! The mod file itself is a .wep file, so you need to assign the weapon on the starting menu by typing `/cwep customwep` or by spawning one yourself. Also, due to some limitations of the modding api, I wasn't able to allow players to assign the sprite on-the-fly in the event that they wanted to use a custom sprite (Ideally this would be done through chat commands, but .wep files do not call the chat_command function)