I talked to Sheldon about this at the London show - I think I said, "I have the Super J and Super P, and it bums me out that I can't really move up to anything else in a few years when I have saved up enough for an upgrade. Do you have any plans for any other basses in the shorter scale?"
If I remember correctly, I think he said there might be a Z series short scale in the future. I can't remember if I asked him about shorter-scale six strings.