A reader up there in the Friendly Giant To Our North solves my previous reader’s problem:

Dear Mr. Derbyshire—I think your correspondent asking about a novel about a computer that takes over a city may be thinking of ‘The Tomorrow City’ by the Canadian science fiction author Monica Hughes. It’s available used on Amazon and elsewhere.

On the general point, a different reader offers this:

Mr. Derbyshire—You say, ‘There’s a real need for some sort of thesaurus-type database for stories, that you could search by plot line.’ I am probably the 67th person to send you the link but here it goes.

He was in fact the first (and at the time of posting, only). Thank you, Sir. This looks like a handy resource, though I haven’t had time to try it out yet.