Thanks a lot, your hint solved my issue! [:)] After adding the d parameter to the proper material within the .mtl file my glass panel is transparent now after importing the cabinet into Sweet Home 3D.
Regarding Art of Illusion: Since Transparency seems to be set in the material properties, I most likely will have to file a bug affecting the exporter there. ... in fact I just discovered there even exists already one - bad thing is, it is already 4 years old [:(]

One question remains to me tough: I was expecting the .mtl file to keep information about the textures used for the model - in my cabinet example a wooden structure which I thought would be reflected in some sort of a texture image. This is not part of the .mtl file since it just looks like this (for the wooden texture called birch):

newmtl Birch
Kd 0.94838 0.90947 0.85077
Ks 0 0 0
Ka 0 0 0
illum 1

So how do I transfer this texture information to the Furniture Library Editor? Should there be an image containing the structure? As a Jpeg? With the same name than the material in the .mtl file?

Thanks,
Michael

P.S.: For some reasons it seems that it is not possible to display pictures located on DropBox directly in the post (or I don't know the correct syntax) - so just follow this link !