Hmm, I'd have to look around to see if that's really documented anywhere - my feeling is that the opening is more of the result of pressure from Kisshomaru than anything Morihei initiated. Of course, the "arms race" started by the Yoshinkan (it actually started with a very successful public demonstration by Gozo Shioda in 1954) provided a lot of the initial impetus.
I seem to recall Ueshiba himself saying it in the audio interview from Aikido Journal, but I'd have to double check to be certain. I was just curious if you might know of earlier examples of him saying similar things or even if there's anything similar going back as far as Takeda? I know Takeda said Daito-ryu was also an art of peace or something similar, but it's always seemed to be more of a response to the recognition Ueshiba was getting than a way of promoting what he was doing. While Takeda seemed to have little problem teaching anyone who had the money, I don't think that's quite the same as "aikido is for everyone", but maybe I'm wrong there.