Michael, you're probably right about the fact that exporting again in OBJ format will generate new triangles... Hexagon does that, Wings3D does that, Blender too, as so does Silo... ;-)

FYI, in Wings3D, you can affect a specific material to polygons to define them as holes.
Although N-gons seems a bit unrealistic, I second the request of Steph3D for what concerns untriangulation. Whenever possible I would prefer to have quads instead of triangles.
MoI objects export nicely in other applications (I tried OBJ) as long as the object is "final". I've had some issues for objects that I want to refine further in other applications (ZBrush for example) but I've not experimented enough to report now.

Coming back on materials, could it be possible to implement "colours" for objects (or faces and even curves...). A basic colour system would do the trick for me, with a few selection options (select objects or faces by colour, hide by colour...)

Enough for today... Now it's time to go and play with MoI !

Cya

Schbeurd