As I am sure your aware David, Han Sutemi (half sacrifice) is any technique going to a knee or sitting, so these would have been done by O'Sensei. More relevant is the hanmi hantachi with an attacker grabbing from behind. From here O'Sensei rolls back under his uke and puts one foot under his chin and the other behind his neck. He then throws him forward. This is in his book Budo, and in the 1936 film.
Graham, it's true that dropping to a knee is technically han sutemi, but I I think the reference is more to the full-body drop from standing. And for that reason, I wouldn't really consider the rolling back into the neck throw as sutemi per se, but you do make some good points there.
Best to you.