So exactly what would the BB be? The Schon STL-11 is certainly a production model, but there's nothing production about a BB one off. If it was totally his idea, design & specs, then it's still a custom cue custom built by him to his specifications. It may not be custom fit to a buyer, but it's certainly still custom. Custom means it was built to fit somebody.....but there's no specification saying for whom it can be built for.
I agree with your notion, but there's no technicality to categorize your feelings. We have custom and we have production, nothing else. 99%+ of the cue world would consider the BB to be a custom. So it's essentially a custom cue by the commonly accepted idea of what a custom cue is.
You have the BB & are playing with it in a room, and some guy comes up to ask about the cue. He asks if it's custom or production. What would you tell him? Would you give him a lecture about how you felt it was neither & explain why, or would you simply say, "custom", so you could get back to playing your game?