Actually, I doubt the custom shop would make it (for me), even if I asked and paid a king's ransom for it. They'd only do that if I was a celebrity. They might do it for Chris!
But it's on my mind and my luthier could indeed glue an SG RI and and an SG Z together. A fretless longscale and a short scale fretted eight string would even see regular action with me, it would be extremely fun and creative to play.