Personally if i was developer of any image processing application i would do this:
Secrets Hidden in Images (Steganography)
That idea can be also used in 3d models (like tiny variations of vertices, or simply their indexes in file).
So i would simply embedded serial number of software that assets are created with. Then it should be quite easy to extract texture or models from game, and see what serial number is embedded there inside. If its from illegal copy, write nice email to developer.
But i think those companies got major income from big partners, indies and hobbysts are outside of their scope, low potential money and big risk for bad pr. That is also reason why microsoft does not really copy protect windows, they rather lose some income and have windows widespread, than get bit more but windows not so popular.