Whenever I have the chance to take ukemi for him I attack hard with the thought, 'you're not getting me so easily this time, buddy,' in my head. Of course, he always does. I find I learn the most from him when I attack with the most sincerity.
I think if it was me I'd give a reasonable attack (it's only a demonstration for others to learn from and their learning is the most important thing), but my mind would be focuses very firmly on how Ikeda Sensei (or some other knowledgeable teacher) felt, how our forces interacted, and so on. As I progressed through my quest for finding out what internal-strength was, I always set my own goals based on the 'feel' of the best (and most widely acknowledged by other experts) expert I could find. That feel is critical to understanding what is going on... but then, too, so is some straightforward practical information on how-to's.