Patrick de Block
And I think it is telling that most pre-war students all had solo-exercises and some post-war ones too.(I don't know what every aikido teacher does or did.) Clearly they felt the need to teach that + outside of the waza.
I don't think Takeda taught solo training exercises to any of his students, and I don't believe those who developed their own meant for them to be a method for developing internal power. I mean, Sagawa probably did, but it is pointless to try and chase his material down because he is dead and did not exactly seed the earth with a legion of devotees. Ueshiba left us the good old rowing drill, maybe that's something.
I am really pretty sure the techniques were the training tools for developing the power.