As these databases are both in constant use, with users that have, how do I put it kindly, interesting workflows, it really isn't possible to cron a backup job.
You can't edit crontab to run a script a few minutes from "now"? (My "postgres" crontabs are replete with commented out ad hoc scripts which get uncommented and rescheduled as needed.)
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Born in Arizona, moved to Babylonia.