[psychonic]

From the bit of investigating that I've done, combined with their exact wording, I believe that it is probably fine so long as you are replacing the whole weapon model if a weapon that the player does own (or is a stock weapon), and that you don't do anything to fake the quality, rarity, stats, wear, etc.

If you have a plugin that replaces the model and link it here, I can tell you empirically whether or not it will trigger their current detection methods.