AHH, I see. So the portal declaration no longer goes in the .scn file, only the .mtl file, is that right? And I guess that means I don't have to give it vertex coordinates either. Hah, I should have tried it out and asked about it here before I did all that work this afternoon!
Awesome, I guess this'll be easier to manage, and it makes more sense in the UI to keep this option together with materials.