I don't have the exact quote at the moment, but Brian stated around January 1967 that he's thinking about releasing something that's just him with piano as B-side to "Heroes & Villains"
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I found the exact quote now
Brian interviewed by Tracy Thomas - published February 18, 1967:
“I’m doing the final mix on the A-side tonight, but I can’t decide what to do on the other side. The easiest thing would be to pull something off ‘Pet Sounds’, but I feel that that would be cheating the record-buyer. On the other hand, I want to keep as much of ‘Smile’ a surprise as possible. I may end up just recording me and a piano-I tried it last night in the studio. It would be an interesting contrast, anyway.”
Ok, so this doesn't have to be "Surf's Up" - my fault. Now it would be interesting to know when Tracy Thomas interviewed Brian.