It may well be that the bona fide uchideshi (pre-war) were the ones who were able to witness the personal training. Same for Ueshiba in the time spent with Takeda Sensei in Hokkaido. I wonder how, without that live in, bath, take care of experience, you would pick those personal training regimes up.
Well, the problem with kokyu/ki practice is that you can't see it very well, if at all. Take Fune-Kogi Undo... that's a fairly obvious kokyu-skills drill when done correctly, but how is it done in most dojo's? I'd be very interested to see O-Sensei's training drills and I've also heard they were pretty extensive, in terms of his private solo practice.
However, the one thing to remember is that the core principles of all the kokyu/ki solo practices is going to be the same. For instance, there's a great book called "Ancient Way to Keep Fit" which shows a number of ancient qigongs/neigongs. A confusingly diverse number of them. However, once you understand the core principles, all of those varying exercise routines simply become decipherable offshoots of the core principles.... even the one where a guy lies in a bed in the same position for 12 supposedly different postures. The point being that it's possible to get a keen insight into what areas Ueshiba focused on, if we knew what his private exercises were, but the general principles are sufficient keys to the door, if we don't have his exact exercises.
Another transmission question to ask might be one that focuses on the early exercises that Tohei taught. At initial time around when he was Chief Instructor to when he was starting Shin ShinToitsu Aikido, he would have undoubtedly used a number of Ueshiba's exercises, if he knew them. We already have some indication of him doing this, as a matter of fact. So perhaps a dual line of research by asking Ueshiba Kisshomaru and by asking some of the early Shin Shin Toitsu guys?
Just a thought.