Honestly, those of us who have to use ECVs would all trade the reasons we need them for standing in those lines in a heartbeat, but no one would do that. So, yes, it might seem unfair to those who don't need the devices, except when they realize having those buses just cut their wait down from 60 minutes to 15 minutes because now the standard buses can fit additional people. And how is it fair that just because someone needs a mobility device they have to wait 5 extra buses? Yet, who complains the people without the mobility devices, my suggestion simply brings waits to be on par for everyone, not giving an advantage to anyone over another.
That being said, I would expect the buses would be full sized buses that are designed to hold probably 6 ECVs per bus and I would expect the same rules apply that each party be a maximum of the person in an ECV and 5 additional, any beyond that would need to wait in the regular queue.
These would only be used when there are an excessive number of wheelchairs and ECVs are going to a single place, so let's say they are 10 ECVs and 5 wheelchairs waiting for Animal Kingdom Lodge, they would send one or two of these special buses to supplement the regular buses that would also belonging the ECVs and wheelchairs, making more capacity available for everyone ultimately. This means there really wouldn't need to be that many of these special buses, they are truly just additional capacity that are designed to get as many ECVs and wheelchairs moved so that space can be freed up in the standard buses.