When practicing I did straighten up one time, and the UKE was about 6 feet off the ground. I think it scared him, and me, so I went back to stooping over.
You can do it on your knees. You see judo guys do that with their KATA GURUMA.
I'd be more worried about my head's proximity to the mat as I spin downwards, though, than falling on my butt...
(Ron! Head throws?! Fie, fie!)