What happens if you restore your keychain from a TM backup?
Answering my own question -- because I simply ran out of time: my need for my secure notes outweighed the risks.

I restored an earlier copy of my entire ~/Library/Keychains folder from a pre-corruption Time Machine backup. This did not restore the "iCloud" option in my Keychain Access "Keychains" panel, but the secure notes all reappeared under "Local Items". After restoring the items, I was able to lock and unlock things in System Preferences, and I was able to log-out and log-in to my computer (after making copies of all the secure notes, and so far I can't say I see any obvious problems.

So in my case, the answer to my question is: it didn't do anything bad, though it will (of course) reset any passwords you may have changed since the backup.