Not sure if anyone else has ever mentioned this before or not, but I had been thinking of a complement to Eric Krewson's gizmo. Definitely not a replacement, but something that can used in conjunction with the original.
Recently I finished up my trade bow which ended up being a bend in the handle style Osage, and I found that the reverse gizmo helped fill a niche. The original marked the belly as usual where the reverse one measures the bend on the back of the bow and let me travel from tip to tip without interference from the string or the tillering tree.
The only downside is that it marks the back where you will not be sanding or scraping it off. So make sure you use a pencil, not a pen. Something that can be cleaned or erased off easily. Maybe a piece of masking tape down the length of the bow' s back would help that.
Again this doesn't replace the original. I used both on my bow and they both basically told me the same thing. But the reverse definitely helped with the handle bend and the flipped tips. I'm not sure but it might also help with character bows when used along with the original.
Thanks to Eric for sharing the gizmo with the world. It makes me wonder how bows were made so well before.