I heard that Ikeda-sensei taught solo training in the past.

One exercise that was described to me sounded like a sitting version of the popular standing pole exercise.

This looks like another - unfortunately, in this clip he does not mention that he is actually reverse-breathing on the inhale - hopefully it was mentioned at some point in time before or after this clip. When you hold out your arms like that and pull in your tummy on the inhale, you can feel the pull on your arms, through the muscle-tendon channels that run through your shoulders:
https://www.youtube.com/watch?v=4hB-knlRDZ8

I have also been told that he has spoken about pushing against a wall or other immovable object and "sending" into it; and that he works a lot on "rotating his insides" (most likely tanden/dantien).

That is how he appears to pull his uke off-balance without visible movement of the hips. His reverse-breath engages the tanden, which in turn exerts a pull through the muscle-tendon channels, and thus... "partner goes". Ok maybe there's more force-vectors at work than just pulling through the muscle-tendon channels. But I found the force vector stuff doesn't work without proper connection within the body.