On the other hand, I'm not sure that taking lots and lots of hard ukemi in such a setting would serve as a necessary prerequisite to learning aiki (there are other, less bitter-eating ways);
Cady - you somewhat conflated several separate ideas ---although you do, in essence, hit the points I was trying to make) - but to make the distinction clear:
1. Taking hard ukemi, as Sagawa suggests (and something that would be, allegedly, unavoidable in his dojo against his or his leading students' expertise, anyway), becomes a conditioning exercise, I believe
2. Taking ukemi is also a training exercise - "receiving body" - and depending on one's level of skill, one could train to the edge of nage's ability and "take" the fall, or counter them (uke becomes nage) - this would be the HIPS aiki training that one could, conceivably, carry out within any aikido dojo.