We took a trip with my parents (both
ECV bound) in 2007 and stayed at WL, and they've stayed at POR/POFQ using the ECVs. Upon check-in at WL they were told that in addition to waiting for the larger boats that if the water level in the lakes were too low that they may not be able to board- it would be unsafe to get the scooter onto the boat. Didn't happen, but could be an issue depending on when you're going.
The CMs did say that if that did become a problem, they would find alternate ways for my parents to get to where they needed to go.