Hi,
I had a good look again, and I think I understand the issue now, and it might actually not be an irrlicht problem:
hiker {l Wrote}:irrlicht sorts the scene nodes by transparency/non-transparency. So try to export the transparent meshes as a separate object (set the properties: type="object', name="some filename" interaction="static" in blender before running the track exporter), and see if this fixes the transparency.
That was actually not correct

Irrlicht registers scene node to be rendered in the transparent and/or non-transparent pass. If a scene node (e.g. a kart) has both, it will be registered in both passes, and only render the appropriate meshes in each of the phases. So irrlicht should be able to handle a model like this correctly.
So I had a closer look at the adiumy model: the problem is that the window mesh is not actually set to be transparent in STK, and therefore it is rendered in the wrong pass. When I quickly hacked STK to set all meshes of adiumy to be transparent, you could indeed see through the glass to the driver

So that's good news

Unfortunately stk only supports setting the material properties (like transparency) for textured meshes, so there is no easy way atm to fix this in stk. But the following should work: can you assign a simple transparent texture to the windscreen? Then you can set the appropriate value in the materials.xml file for that texture, making the window transparent, and then triggering the correct rendering order. Then adiumy should work as expected.
There might be some support in the b3d file format for that, but I don't think the exporter can handle this atm.
Could you test the approach with the transparent texture and let me know how you go?
Cheers,
Joerg