Aha, opening the huge (32 MB) OBJ in Wordpad found it held a path to supplied MTL file (30 KB, with 468 numbered materials calling ~140 texture maps !!) but in a deep, deep sub-folder on author's D: drive. Trimming path back to just calling the MTL file by name resolved that. In fact, took longer to re-save than edit...

But why is there no obvious way to 'browse' for a strayed MTL within Poser ?? It would also be a real-neat, portable way of loading scene texture variants...