Check out
http://gargas.biomedicale.univ-paris...cal/index.html
particularly "Angels ukemi Korner"
http://gargas.biomedicale.univ-paris...l/writing.html
Actually, I've just checked and the precise link I was looking for is at this moment broken, but essentially this guy Angel Alvarez, a fifth Dan (I think he trained with Yamada Shihan for years and is currently living in Spain) had his e-mail address on the site with an invitation to send him questions about ukemi.
Perhaps you could mail the webmaster of the page, or perhaps the link will be back up soon.
andrew