So you want an isometric world, but with your character 'flat' against it from the viewer's perspective? Wouldn't that give the character a 45 degree tilt or something?

Regardless, I don't have the necessary knowledge to build a 3d engine to do this, but I would assume that an isometric viewpoint would be enough - you could just add your character and make it 'look' normal and not isometric? The effect would be the same?