I was at a seminar in Kumamoto some months back and received a few corrections on my stance from Hitohira Saito Sensei. By way of background, I have been practicing Iwama style since the early 90s and have seen numerous little tweaks and changes come and go over the years.
Sensei told me to keep my feet closer together than what I had originally been trained to do and my knees bent, lowering myself down. The butt out position was a result of the deeper closer stance. And it was also not so much just the rear out, it was also the belly out and sinking down. Sensei said, and I'm pretty sure I understood this well enough, that this was a way to train roppho, the infamous six directions of much debate. Hmmmnn.
The man could move, no question of that. I ended up flat on my back often enough to prove that to myself.
Now, by coincidence, I have been looking up a lot of information recently on bayonet technique, both traditional European rifles form and Japanese Jukenjutsu, looking for some connection with Iwama weapons. I know of no Aikido teachers who regularly train with Mokuju, the wooden bayonet, so I became curious.
It seems that sometime in the 1890s the Japanese military changed the style of bayonet training to a form which utilized spear techniques from different Japanese ryu. Now, I've looked at photos from that time and can't really see any radical difference in stance or attack between Japanese and European/American bayonet methods. In general, Japanese techniques tend to a more upright posture, and from World War II and onward American methods prefer a crouching position.
That said, I did notice in some pictures of the American bayonet a distinct rear end out approach to the thrust. More noticeable in the use of shorter carbines equipped with bayonet than with the longer rifles of old. As an Iwama guy, I find that kind of interesting.
I haven't found a solid representation of Jukenjutsu with the rear out just yet. Some images in Google image search hint at it in bouts but just as many feature a straighter more upright posture.
Additionally, I have seen pictures of Ueshiba doing the tail out position in technique, used as proof of Iwama stance. I have also seen plenty of pictures of him standing upright with no twist at all. As an Iwama practioner all I can say at this point is go figure.