Restore iPhone SMS history from Mac Messages without iPhone backup
This will surely frustrate later arrivals who only have chat.db available, but I later realized I have other iOS devices with up-to-date Messages. I extracted sms.db and Attachments from those backups (using iPhone Backup Extractor), merged them into my latest iPhone backup (using iBackupBot), and restored the backup. I'd suggest turning on airplane mode to avoid receiving texts in the backup/restore interim.

This tutorial provides details on where these files go.