We had a course recently with a guy who spent the weekend on tenchi and aikiken mainly, and the simularity between them. (Malcolm Shewan, 6th Dan in Aikido and Iaido, very very good, and I'm not going to pretend I can transfer a lot of what he said cos I can't.)
One thing he was doing, which I found unusual, was working on what you're calling the sky hand. In one of the exercises we were simply raising it up _WITHOUT_ putting it outside ukes hand, if you see what I mean. He went to pains to point out that in a real situation they'd simply let go. We'd raise that hand straight, uke would be unbalanced (foot would raise a little), we entered, placed the other hand on the back of the neck, and downed the person. We also did it with uke executing a chudan-tsuki with free hand- I mention it because the step brings you just outside the punch, and it'll help you see what I'm saying.
We did exercises also where people held down the bokken, and we found how to raise it correctly, etc. So basically there was a lot of exercises to teach the principle of how to raise the hand.
And so to my point. I was explaining to somebody what we'd done, and got them to do katatedori. Because I knew I'd probably break the grip, I put my other hand on and raised mine (fairly gently). The unbalancing effect was pretty dramatic, and sure surprised me.
So I'm just hoping I explained well enough that you might be able to repeat some of the exercises we did. I hope it helps you with the raised hand. (I often find where I personally have difficulty raising a hand, it's because I'm pushing into somebody where just going straight up would work better.)