Please note that the info on this and the Final Fantasy IV page is out of date. I'm too preoccupied to rewrite it at the moment, so go check out the November 2003 Elusions at Lost Levels for the updated story.
Yes, this is from the same Seiken Densetsu series that was released in the US as Final Fantasy Adventure/Secret of Mana. For those who don't know, DOG (Disk Original Group) was a branch of Square that specialized in FDS games.
I have no idea how close the gameplay would have been to the eventual GB/SFC releases, but I do know the game was going to be huge. Everything seems to point to the game being released on five FDS disks, which, according to KingMike, would put it at 5 megabits, just under the largest licensed US game, Kirby's Adventure, which used 6 megabits.
Also, TheRedEye has gotten word that a collector in Japan actually owns a copy of this.
