My brother had one made as an interior door to separate the workroom from the showroom in his retail shop. He used the existing door, had it cut, hinges added and a small shelf put on top of the bottom door so he can lean on it and chat with customers. As an exterior door, depending on your climate, I imagine weatherstripping would be difficult if not impossible. The company linked below makes custom doors and that may be your best option. Here is a link that might be useful: Lemieux Doors

Haven't done it but your post brought back memories of my parents having one years and years ago on our back door. There is a site on the internet with instructions for the conversion - you have probably already seen it. Take some before and after photos for us. (And welcome to GardenWeb!) Here is a link that might be useful: Dutch Door Conversion

The door slab can be used for the barn door, but you'll want to remove the hinges and patch/paint them. The knobs could be left on, but realize that they'll prevent the doors from opening the full width of the opening (may not matter to you). And you'll have the latch mechanism that will be extraneous. Here's a photo of a barn door setup we used between a bedroom and bath. We still had normal molding (casing and baseboard), so that wouldn't need to be removed. Your doors will just cover the opening (like they do now) without any overlap. Also, the door jamb will have mortises to full in (where hinges were), an extra piece of wood in the center of the door jamb (don't know the name) that doesn't exist for wall openings without doors. Those could be removed. As long as the above don't bother you, it can certainly be done.

"Decks are required to be designed to 10lbs/sqft dead load and 40 live. Whether your roof has to meet that depends on where you live. If you live in snowy areas, you'll be fine." - Does not compute. First, you do not figure roof loads the same as deck loads. Live loads on decks move - often in unison - such as dancing to music - which introduces a destructive sine wave like a tsunami into the structure. Second, even roofs in snowy areas on the 47th parallel only require 15# dead load and 30# live/snow load. (St. Louis is only 20# live/snow) For that theory to remotely work, you would need to be in a VERY snowy, ski resort type of area - AND keep the deck shoveled in the winter to compensate for the extra 10# dead load of the deck - unless the original design had already foreseen and allowed for such use. The 50# total design load required for a deck does not fit on a roof designed for only 45# total (35# total St Louis) Even the 3# added by the PliDeck system can overload the design values, creating sagging, or creap, over time - causing puddling - leading to other problems. A roof , as in your example, would also require railings, (waterproofing problem) and may not be as flat/level as it looks like from the ground.