Came to this thread very, very late, but a couple of thoughts...

Some modelling apps hard-code MTL's path into OBJ, so it is not accessible without a hunt. Fortunately, Windows' bundled Wordpad will open even huge OBJs. You can watch the % grow as load progresses, then the MTL reference will be visible in first few lines. Carefully edit out extraneous path stuff, save with a smile.

Similar issues with MTL files, whose map-files may have both hard-coded paths and quotes, all unwelcome. Notepad will do for this clean-up...

( One of these days, I'll figure the 'mapping' between MTL and MC5 ...)

None of the referenced texture files included ? Yeah, met such. My kindest take is author suddenly realised those lovely textures were not a 'merchant resource', could not be 'sold on' without getting an extended license. Often, I suspect author completely forgot those files were in a 'library' far, far away from WIP's Runtime...

I've had some luck using eg Google Images to find those named files. Sometimes, folder tree names give a clue to which on-line free texture library contributed. I have resorted to contacting authors, asking what a particular BMP resembled, and what size. Often, I've had a reply with links or attachments...

One wry 'gotcha' is when you import an OBJ and Poser finds its MTL, then prompts for texture files with different, shorter names to those supplied.

This bug, which I've called in, dates all the way back to P3/P4. Just 'left arrow' in the prompt field and the leading characters of name will show up, too. ;-)