I've been contacted via PM by someone who wishes to remain anonymous but who was at that particular seminar, which was in 2000. They have given me permission to paraphrase their PM to use in this post.
The theme of the seminar was, in fact, how to change the angle at the point of contact to weaken the uke's upper body by either rotating (i.e. supinating and pronating) your forearm or by bending your elbow to disengage their shoulder.
Thank you for the additional comment. The information you provide fits well with my memories of what Ikeda Sensei was teaching at that time...your sentence "how to change the angle at the point of contact to weaken uke's upper body" is something I remember Ikeda Sensei talking about a LOT in those years.
Your cautionary words about watching video clips out of context make perfect sense to me.