Yes, I found that "Scene file format" page yesterday, it's useful but limited, nothing in there about portals.

I came across 1 note in the XSI exporter wiki page to add PORTAL to a material to change it into a portal, not sure if that was meant literal though or just as a placeholder..

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB