Your post intrigues me as an interesting engineering question, and I think of my father-in-law who had a fully-calcified ankle. Ankles are such a crazy joint. I'm sure, if you're here, you've already done a lot of looking. Seems like Ampt Biking out in Colorado would be an interesting phonecall. Van Raam is another. Freedom concepts. I specifically suggest calling places like this because they're always a "clearing house" for information and "yep, heard that before," is the conversation you want, right? You cite "2 custom builds" in your profile, so this is not your first rodeo and you've probably heard enough about recumbents and hand cycles and the like. I would call Calfee Design, they are an interesting place. Another "off beat" angle might be places like Winter Park, CO, where they specialize in adaptive skiing, and now appear to focus on a wide variety of adaptive year-round -- not that you want to go play there or anything, but you want to tap into their network - "We know a shop out here that does..." is your holy grail. Bicyclecolorado has an interesting page
here . Post up with your progress, I think your journey is interesting.