The typical Herse decaleur attaches by the stem bolts to the underside of an Herse stem, with the bolts going vertically (perpendicular to the ground).
One downside, though, is that this results in a bag that is mounted pretty high. We could modify the bag attachment, or perhaps increase the drop of the decaleur, to use a smaller bag. So with the current arrangement, for a frame that is essentially like a 59cm, with an inch and a half of drop between saddle and bar tops, the necessary bag is a Berthoud 28.
So if this design has some appeal, let us know and maybe we'll run a batch. We probably would use a maker somewhere in Colorado for this, but probably not Mark Nobilette as he is so busy.