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Keys in the keypool store when they're generated (and the oldest are always used first). Asking "what's the oldest key in the key pool" seems reasonable, and I think it would give you what you want-- you could compare that timestamp to the timestamp of your last backup to see if you're covered.
Although if you're going to periodically check to see what the oldest timestamp is (or you're going to periodically check to see if bitcoin has written a new timestamped backup file or periodically check WHATEVER) then it seems simpler to me to just periodically always call backupwallet. Disk space and bandwidth is cheap these days...