Eureka! Yes, now I have determined that I can edit either the .mtl file as you described above, or the .obj file, using a standard texture called glasstransparent. I ended up doing the latter, producing a great pair of shower doors for my tub! It would be nice to have the transparency be carried directly into the exported object by AOI, and I will suggest that, but just to be able to achieve it so easily makes me very happy right now. Many thanks for your help and references!