I think the idea of causing as little harm to an attacker as possible is great. At the same time, I think this probably gets very tricky in real-life situations where you're getting attacked by one or more persons who really want to hurt you. I think this is an issue of ukemi- even if you can execute technique well/not lose your cool, what are the chances the attacker will know how to receive a technique?
A lot of times when I train with newbies, they get confused as to how to get taken down by a technique (and that's gotta be half my own fault, I'm pretty new to Aikido, but not just my fault
). Shihonage, for example- if uke doesn't know how to receive it smoothly, there's the possibility of dislocation, or getting thrown down really hard, or both. Good to keep in mind that throws are not in themselves more "harmonious" or peaceful than strikes. A hard throw even onto mats can be jarring. Imagine a hard throw onto concrete, broken glass, down some stairs, etc. I'm not saying that it's impossible to use Aikido for self-defense with a minimum of harm to the attacker(s), just that it is probably really tricky, a lot trickier than dealing with a more or less cooperative uke on the mat. The interesting thing there, I think, is that you might be presented with the choice of either getting out of the way (using footwork to evade), or try for some serious technique, probably with atemi. It might be more in line with the idea of not hurting the attacker to just walk away. Of course, that's not always possible (and it's a whole different story if someone else is being threatened).
I know this doesn't address the question of where the minimal harm idea came from, but it's something that came to me last summer, and I thought it was at least partially relevant.