No, but I use some of the things Ellis taught in it. I don't remember Ellis teaching specific ukemi for koshi at the seminar, but I do think what he taught can be applied there, and it seems to work.
Find someone who is good at koshi, and try his methods out slowly. I think you'll find some nice opportunities. Remember the counter he taught for iriminage?
A lot of my ukemi is yoshinkan based, with my own little ideosyncracies thrown in (like from stuff I've learned from Ellis at the seminars and other places). Some days it just depends on how I'm feeling too...I'm not 43 anymore!
I agree with some of the statements here and in the other thread...someone not good or not comfortable with koshi can mess up learning the ukemi. But once you've got a pretty basic method drilled in to just protect yourself, if you grab a good partner, you can find all kinds of interesting things. Anyhow, I tend not to koshi so much as to cut kesa giri with their arm so I don't have to load anyone on my hips. Loading bad when you want to stay mobile. Cutting good....and controlling an arm as in shiho, yonkajo, or even better, jujinage is even better. Less chance of giving your back and getting choked out!