I am not sure that the limit on going frameless is length as much as it is displacement.
As a boat gets heavier, it needs more sail area if it is going to sail well. If the boat is going to stand up that bigger sail plan then it needs more stability. The combination results in the keel connection loads and rigging loads increasing. And as these loads increase, it gets harder to disburse them into the skin without distorting the skin, and so load patches need to be added. (Think of it this way, a sail is a single membrane monocoque structure, and so as the sail gets larger bigger stress patches need to be added at the tack, head, clew and reef points.)
Like the sail analogy, on a small boat you might be able to simply use heavier hull plating (sailcloth) rather than add stress patches but as the boat gets heavier the concentrated loads increase more extensively than the skin loads.
At some point, merely adding stress patches aren't enough and so knees and a minimum amount of frames get added at the highest stress locations rather than end up with a hull that is absurdly thick. Eventually as the boat gets heavier still these added frames end up concentrating loads in narrow bands and so additional structure is added to move the loads more effectively around the skin.
A good example of this phenomina is early fiberglass boats. When they first started building fiberglass boats, they wanted them to be frameless. The US government had done a lot of research on FG during WW II and the early designers knew that fiberglass was very string in bending, but not very stiff. Unlike the mythology, the early FG boats had thick hulls to deal with flexure. (Unfortunately to get that thickness early boats generally used a number of methods to bulk up the laminate which undermined the strength of the laminate especially over time. but that's another story..)
When you look at these earliest FG boats, even though they were very heavily built, it was not unusual to see dimples in the topsides where rigging loads distorted the hulls. Pretty quickly designers and builders started adding knews and structural bulkheads. By the mid to late1970's fully framed FG boats became the norm.
It is a similar problem with steel, as the boat gets longer and heavier, at some point the choice becomes to add frames, end up with an absurdly heavy hull, design a way to distribute the loads with the geometry of the hull and/or deck, or live with distortions.
There actually have been production aluminum power craft. Probably a decade or so ago, I attended a lecture on new aluminum Coast Guard boats. The aluminum sheets were precisely cut by a computer driven cutter. The electronically developed plate drawings allowed the yard to develop very efficient 'nesting plans' which allowed the carefull layout of the individual parts on the plate so there was very little waste. The cut plans included small tabs which allowed the precise aligment of the parts as the boat was assembled. Bulkheads served as part of the structure and also helped precisely control the final plate shapes. Very impressive to see.
I think that aluminum lends itself to the newer hull forms and so is more popular in Europe where they have embrassed the Open Class style boats more than we have here.
A couple of very successful origami aluminium 55 footers have been built here. I saw one at Christmas Islands the last time I was down there. She sailed back home to northern BC in November, a severe test in itself. The owner, Jean Marc loves to sail his homeland , Hecate Strait, year round, and the rougher the better for him .He has done enough miles of fulltime cruising up there to go around the world . You can often find him in Old Bella Bella, where he bought the place.
The French have been building heavily plated aluminium origami boats for a long time now, called "Strongall boats." A google search will find them.
I always use some transverse floors on my boats to help stiffen the keel attachment . On the single keel there is the aft end of the ballast and centreline tank , nearly 4 feet wide, and going to the bottom of the keel. Then there is a baffle 4 feet forward of that, again 4 feet wide and going almost a foot into the keel . Then there is the forward end of the tank and the top, most of a 4x8 ft sheet of steel .making a super strong and rigid structure . Twin keelers have 4- 3x3x1/2inch angles, on each side, from the chine to the tank edge, supporting the twin keels.
The topsides are a 3 ft wide, 3/16th steel plate on end, giving a chain plate attachment far stronger than any rigging wire you could ever put on them.
A good comparison of stiffness is to consider the way some GRP boats have cheated the rating rules ,by putting a big hydraulic backstay adjuster on, and cranking it until the boat sags and the waterline shortens considerably, for measuring. If you try that on a steel or aluminium origami boat , you wont change the waterline length a sixteenth of an inch. You will break the backstay long before that happens. I once broke the handle off a 3 ton come along trying to take a 1/16th inch twist out of a hull, after the decks were on. The hull never budged.
I put a half inch plate web under the mast support, then connect ( triangulate) the ends of that to the chine with pipes. The chines, structurally, constitute a longitudinal angle, 3 ft by 5 ft by 3/16th, far stronger than any combination of rigging wire which will ever load them
60 ft seems to be the upper limit, but one can use origami methods on larger hulls, and put stiffeners in after the hull is together, saving a huge amount of time over traditional methods.
A mold wouldn't give you any advantage over the external frames Van De Stadt uses. That kind of framing type mold would not be worth the trouble for a one off ,but would be well worthwhile for mass production. The weight of the steel would push it down into the frames, little more pressure is needed. A similar mold for decks would also work. Putting the plate shapes on a disk, for computer cutting, would save a lot of time for mass production. Interiors could also be done that way.