There hasn't been any solution to this anywhere I've looked. The .plymodels are apparently exclusive to the game itself, but it runs on Unreal Engine 4.18, so they must be some form of asset related to that. I'm not sure exactly how, but it seems the game converts it to a proprietary format when imported from workshop, The problem being, if you lost the source files, you can no longer update your own creations. I've wanted to study a few workshop models, and a friend of mine actually ports to it but lost his source files to a hard drive failure. I was hoping someone here could find a way to extract the models and textures from these files so we could restore a few things. I picked a random downloaded model from my workshop folder for study, since it's all just numerical values for names and I don't know which is which.
https://www.dropbox.com/s/dhvc03ckkh0cl ... t.zip?dl=0