Peter, I'm also interested to find out when the stance developed, but I don't think what you're seeing in that Daito-ryu video is a Yoshinkan kamae. In terms of posture and movement, the guy at 1:40 bears little resemblance to a Yoshinkan kamae--it's only that he has two hands in front of himself at different heights that makes it look somewhat similar. To me, it looks more like a stylized attack used for kata purposes--see how the guy at 1:53 has his hands together before he attacks.
In the 1930s video of Ueshiba demonstrating, I don't see anything that looks like a Yoshinkan kamae, but I have not watched that video closely.
Another source of early information is a book that Stanley Pranin has referred to--there is apparently a very early instruction book with photos of Ueshiba demonstrating--one could look here if copies of it could be found. I don't remember the title, but Pranin tells a story about showing it to Morihiro Saito and having Saito be very pleased that looked like the aikido he was teaching.
That said, I do think there is more to the history of Yoshinkan teaching methodology than what the common stories tell us. See, for example, my post on "forerunners of kihon dosa":
http://aikidomugenjukukenshusei.word...of-kihon-dosa/
(Background info--in Yoshinkan today, I never see people doing funakogi undo)
about Yoshinkan kamae
I don't remember what Total Aikido says about kamae, but I would say three things about it:
(1) As far as I can tell, if you do kamae correctly, it includes pretty much all the "postural information" that you need to do Yoshinkan aikido. (For example, strong back leg, shoulders down, hips aligned, relaxation, center forward, balance, etc). Whether it is weak from the side (it is, very) is kind of irrelevant--it is not a "sparring posture". Within the basic outline of the posture there are many ways to stand by contracting or relaxing different external and internal muscles. So you can actually do solo training by just standing in kamae and experimenting on yourself.
(2) I have discovered that the key to solving the riddle of many techniques is simply to do kamae. This is, for example, one way of conceiving of nikyo in Yoshinkan--once you have uke's wrist, you just return to basic kamae and they crumple if your kamae is correct.
(3) Payet-sensei repositioned me once and had my training partner push on me as hard as he could. I was really totally relaxed and just with correct balance and posture, I was immovable. This was something I cannot recreate by myself, but it felt amazing--very different from aikido's unbendable arm trick. Also, I felt all the power in my body was centered in my big toe, which is something Shioda mentions in Aikido Shugyo, but which it's difficult to feel normally. Usually, you end up pushing with your toe or gripping with it, or something else that involves incorrect application of strength.
For another Yoshinkan shihan who does not always stand exactly in textbook kamae, see Ando-sensei:
http://www.youtube.com/watch?v=MzEN9LBP2sA