Dang. Two worlds of hurts back to back!
I agree, though. Getting seamless branching working (thanks again, Williarob) was a challenge enough. Getting a BD-J disc authored and working would be a nightmare, as cool as those picture-in-picture features are.
I think PiP doesn’t need BD-J, since the Japanese Studio Ghibli Blu-rays have no BD-J and they have PiP features in the Storyboards (essentially watch the workprint version of the film where it’s entirely storyboards) and the discs are not BD-J.
The PiP feature in the Storyboards in the Japanese Ghibli Blu-rays is that the workprint takes up the entire screen, you can choose whether the final film can be seen in a smaller window and you can even choose which corner to put the final film’s window.
The US Ghibli Blu-rays, however, don’t have the PiP feature for the workprint storyboards.
And while most people don’t like watching the workprint feature which is a standard for most Ghibli releases, I actually love watching the storyboard versions. I love watching workprints of films, and I really love it when they include a workprint as an extra on the official Blu-ray or DVD.
But I’m getting sidetracked. What I’m saying is that if the Ghibli Blu-rays to get PiP working without BD-J, then it’s theoretically possible for most.