I don't see how they can restrict ECV's from going into Downtown Disney. My last trip I had no problem at security checkpoints with my ECV. I stayed at the GCH and used the security exit into DTD (as well as the security entrance into DCA). They will ask you if you can walk through the metal detector, but if you can't they will still let you in. Not sure of the exact process if you stay in your ECV, but I wouldn't be surprised if they have a wand to check you like at airports.