(Several times Ushiro sensei said he didn't do anything to the attacker because he would have had to harm him/her. Despite this sounding like "Drink my kool-aid" I think he was talking about the attacker (seminar participant) charging in with tunnel vision.)
Sometimes developing proper sensitivity is better done the hard way. Vladimir Vasiliev was working with a fellow at one seminar and moved as if to strike his partner. The energetic intention to create movement was there for anyone watching to see, yet his partner didn't react at all. So, on the next movement Vlad actually gave him a hit. Low and behold the next time he threw the same strike, the partner was moving.
This goes beyond the simple idea of negative reinforcement... it hurts if I stand still so I will move.... The pain involved with taking a hit of this type actually stimulates the nerves and begins to develop a body based sensitivity that allows one to feel a blow coming before it arrives. Since most of us in Aikido do not actually strike our partner's when they are unresponsive, it is more difficult to develop this type of sensitivity. I have seen Ushiro Sensei totally let an uke off the hook rather than hurt him and then the uke went away thinking that Ushiro Sensei hadn't been able to do anything to him. Everyone watching could see that the fellow was simply too full of tension, mental and physical, to perceive what Ushiro was putting out. The only option was to hit the guy and Ushiro chose not to.
I do not think that this type of training should be too difficult if the proper uke / nage relationship is trained. It just can't be some kind of competition... that won't work. But if both party's treat the training interaction as instructional and that their purpose is to facilitate their partner's learning, then I think it can be done properly.