I know we're Monday morning quarterbacking this, but I think a spoke wheel truss rod would make it way easier to get the headstock size down to something manageable. And I'm digging @Bearitone 's string lock idea. The thing I hate about headless designs is that so much care is taken in the design, and then you see the string locks, with little bits of string sticking out, all cut at varying lengths, and think, "Welp, I guess they stopped caring about design when they got to this step." It might be cool to see a headstock that has a whale tail sort of design, like the Ritter Jupiter, but on the head end, and much smaller. But that way the string locks (or whatever the mechanism is) would be hidden behind it.