An experiment that I did just now:
I copied a folder containing a Waveform OBJ and its textures into the RuntimeTextures folder, and imported the OBJ. That caused the texture references to be reasonable for the prop. I saved the prop to the library and restarted Poser. The prop loaded with textures in place, and the references look correct in the PP2 file. I created a polygon group and spawned a prop. The new prop was correctly textured.