I only partially followed the discussion about the marker lights on the cabins, so these ideas may have already been brought up, but could they indicate a cabin that's headed for the stationary load/unload platform? Since an arriving handicap cabin would have people remaining inside it as it crawled through the unload section of the station, it would be a visual signal to the unload CM not to hassle the guests to step out. And likewise after the cabin was dispatched from the stationary load, it would have to crawl through the load section of the platform. An indicator light would indicate to the loading CM not to load anyone from the standby line into that cabin.
Or perhaps they designed the Riviera station to only open the doors of cabins destined for Riviera and the lights indicate to the CM loading the cabins who to load into that cabin? (Though doing that would require two separate queues for the Epcot line)
-Rob