Actually, it is VERY much like the sound folders, almost exact same in general concept.
Texture skinning's actually pretty simple in theory, and as we can see in the case of the terrain editor, the 3D model uses both exterior and interior depictions by simply using textures in BMP format. If it was something hard-modelled like the cockpits in IL-2, it would be 'integral'.
Thus, the problem is when the terrain is loaded, the 3D model chooses different BMP textures to "paste" on to the polygons. The exterior textures are pasted individually, according to planes, while the interiors are pasted with only one generic source. If HTC can designate the interior texture source to be also pasted in a plane-specific manner, the problem can pretty much be easily solved.