I was testing out your findings here with my hack and I ran into something interesting. It seems that for most of the locations, there's a set, unchangeable "direction" for the ferry that isn't affected by using setvar 0x8004. You can only change the the direction if you're using 0 or 1 as the location, but no matter what I did I couldn't get the ferry to travel left using 2 - 7. Not sure if you're aware of that or not, just something I thought was interesting (and maybe you know of a way to disable it? :P)