Was going to post this to the other thread but the discussion seems to have got a little warm and it's been closed by a moderator.
Anyway was thinking about this style of fit and discussing it a bit on uk.rec.cycling.moderated. The general philosophy seems to be a straighter back, less weight on the hands, and a high seat (with probably a toes-down pedalling position) because this avoids stressing the knees. Pressing hard with bent legs is bad for knees.
The downsides of this position are aero, the high centre of gravity, and a lot of weight on the seat. The high CoG limits braking on dry roads especially downhill. More weight on the seat is not a problem in itself-- after all it might be better than putting all that weight on your hands-- but it does mean you want a wider, squishier seat, perhaps sprung, and this can get in the way of pedalling.
The solution to all of these problems is a slacker seat-tube angle. It gets the centre of gravity lower and further back, so braking is safer. It means you can have a wider seat without it disrupting pedalling since your legs aren't fighting with the sides of the seat. It means you can put your foot down easily when you stop in spite of the long seat-tube. It's even slightly more aero.
It turns out that there are not "upright" bikes and "recumbent" bikes but that it is actually a continuum, a bit like LGBTIQ. See
RANS Bikes for some interesting frame (and seat) designs. Old Raleighs from the 40s and 50s also had a much slacker seat-tube angle, perhaps 68 degrees or so, and sprung seats, and a swept-back handlebar. I see no reason why a bike like this should not be very comfortable over long distances although a bit slower for aero reasons. I wonder to what extent this could be mitigated by the addition of a small fairing.
One disadvantage of the full recumbent design is that you can't stand up on the pedals a bit when you hit a bump or a pothole, especially important for off-road or gravel. The sweet spot may be around 68 degrees or so. Playing around with frame designs I realized that if you go slack on the seat tube you start to need very long chainstays. So why not make the back wheel smaller? Then it occurred to me that this is a hack you could do without even making a new frame. I didn't see this at first because for me the fun is always making a new frame. Throwing a 26" wheel on the back of frame intended for 700c, which an equivalent low-profile, i.e. road, tyre, should slacken the seat tube and head tube by about 4 degrees. This should keep the steering geometry manageable without needing a custom fork, especially if you start with something with quite a steep head-tube angle. Then you'd add the big wide seat. The handlebar is automatically higher at this point but you might want to find one that is more swept back. Maybe this is something Moisture could try out.