I've always found the search engine quite clunky, and of questionable reliability. I think an actually explicit social norm will solve most of the problem. That said, I won't be put out if posting rationality quotes is not worth my effort.
So far as I know, the rule is just that a quote shouldn't have appeared in a quotes thread, but if it's appeared elsewhere, it's ok to post it in a quotes thread.
A cached thought: We need a decent search engine, and the more posts and comments accumulate, the more we need it.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.