That works. I didn't know that objects with d 0.0 were still exported, they are 'invisible' after all.
It's not perfect, you get an extra material property 'invisibleBox' that has no use. Because of the specifications of the obj/mtl files you cant't change it into '(invisible)' or 'N/A', so I have to resort to something like 'NA', 'nothing', 'notused', or 'filler'. I haven't decided yet.

It does solve the bounding box problem and that was the most important.